It has been so long since I needed to update the Tabs on my Account page that I am having trouble with the code. Can you please review my code and let me know why the Object GAI Opportunities is howing the above error.
<apex:page standardController="Account" showHeader="true"
      tabStyle="account" >
   <style>
      .activeTab {background-color: blue; color:white;
         background-image:none}
      .inactiveTab { background-color: lightgray; color:black;
         background-image:none}
   </style>
   <apex:tabPanel switchType="client" selectedTab="tabdetails"
                  id="AccountTabPanel" tabClass="activeTab"
                  inactiveTabClass="inactiveTab"> 
      <apex:tab label="Details" name="AccDetails" id="tabdetails">
         <apex:detail relatedList="false" title="true"/>
      </apex:tab>
      <apex:tab label="Contacts" name="Contacts" id="tabContact">
         <apex:relatedList subject="{!account}" list="contacts" />
      </apex:tab>
      <apex:tab label="Open Activities" name="OpenActivities"
                id="tabOpenAct">
         <apex:relatedList subject="{!account}"
                           list="OpenActivities" />
      </apex:tab>
      <apex:tab label="Activity History"
                name="ActivityHistory" id="tabActiHis">
         <apex:relatedList subject="{!account}"
                           list="ActivityHistories" />
       </apex:tab>
       <apex:tab label="Contracts"
                name="Contract" id="tabContract">
         <apex:relatedList subject="{!account}"
                           list="Contracts" />
      </apex:tab>
      <apex:tab label="Travel History" name="Travel"
     id="tabTravHis">
         <apex:relatedList subject="{!account}"
                           list="Travels__r" />
      </apex:tab>
      <apex:tab label="Opportunities" name="Opportunities"
                id="tabOpp">
         <apex:relatedList subject="{!account}"
                           list="opportunities" />
      </apex:tab>
       <apex:tab label="Cases" name="Case"
       id="tabCases">
         <apex:relatedList subject="{!account}"
                           list="Cases" />
                           </apex:tab>
     <apex:tab label="Underwriting & Service" name="Underwriting_Service"
     id="tabUndeSer">
         <apex:relatedList subject="{!account}"
                           list="Services__r" />
  </apex:tab>
  <apex:tab label="GAI Opportunities" name="Opportunities_GAI"
       id="tabOppoGAI">
         <apex:relatedList subject="{!account}"
                           list="Gaiopps__r" />
      </apex:tab>
   </apex:tabPanel>
</apex:page>
Addtionally here is the screen shot from my Relataed To Account field:

I am trying to create a Tabbed Account page and keeping getting the following Error:
Activity History' is not a valid child relationship name for entity Account 
I have tried several of the recommendations given but nothing is working. Can you please review my code:
<apex:page standardController="Account" showHeader="true"
      tabStyle="account" >
   <style>
      .activeTab {background-color: #236FBD; color:white;
         background-image:none}
      .inactiveTab { background-color: lightgrey; color:black;
         background-image:none}
   </style>
   <apex:tabPanel switchType="client" selectedTab="tabdetails"
                  id="AccountTabPanel" tabClass="activeTab"
                  inactiveTabClass="inactiveTab">  
      <apex:tab label="Details" name="AccDetails" id="tabdetails">
         <apex:detail relatedList="false" title="true"/>
      </apex:tab>
      <apex:tab label="Contacts" name="Contacts" id="tabContact">
         <apex:relatedList subject="{!account}" list="contacts" />
      </apex:tab>
      <apex:tab label="Open Activities" name="OpenActivities"
                id="tabOpenAct">
         <apex:relatedList subject="{!account}"
                           list="OpenActivities" />
      </apex:tab>
      <apex:tab label="Activity History"
                name="ActivityHistory" id="tabActiHis">
         <apex:relatedList subject="{!account}"
                           list="Activity History" />
       </apex:tab>
       <apex:tab label="Contracts"
                name="Contract" id="tabContract">
         <apex:relatedList subject="{!account}"
                           list="Contracts" />
      </apex:tab>
      <apex:tab label="Opportunities" name="Opportunities"
                id="tabOpp">
         <apex:relatedList subject="{!account}"
                           list="opportunities" />
      </apex:tab>
       <apex:tab label="Cases" name="Case" 
       id="tabCases">
         <apex:relatedList subject="{!account}"
                           list="Cases" /> 
     </apex:tab> 
     <apex:tab label="Underwriting & Service" name="Underwriting_Service" 
     id="tabUndeSer">
         <apex:relatedList subject="{!account}"
                           list="Underwriting_Service" />
       </apex:tab>
       <apex:tab label="Notes and Attachments" name="NotesAndAttachments" 
       id="tabNoteAtt">
         <apex:relatedList subject="{!account}"
                           list="CombinedAttachments" />
      </apex:tab>
   </apex:tabPanel>
</apex:page>
Thank You - Julie

I need a formula that will calculate the difference between Projected Fees and Actual pro-rated Fees but does not include our first year accounts. I have tried the following: 
IF (First_Year_Account__c) = "false"
     Pro_Rata_Agreement_Fee__c  -  Projected_FY_Fee__c

 I get the following message: Error: Syntax error. Extra Pro_Rata_Agreement_Fee__c

 The First Year Account field is a check box that is only checked during the first annual term of the account.
Any help would be appreciate.
Julie J

It has been so long since I needed to update the Tabs on my Account page that I am having trouble with the code. Can you please review my code and let me know why the Object GAI Opportunities is howing the above error.
<apex:page standardController="Account" showHeader="true"
      tabStyle="account" >
   <style>
      .activeTab {background-color: blue; color:white;
         background-image:none}
      .inactiveTab { background-color: lightgray; color:black;
         background-image:none}
   </style>
   <apex:tabPanel switchType="client" selectedTab="tabdetails"
                  id="AccountTabPanel" tabClass="activeTab"
                  inactiveTabClass="inactiveTab"> 
      <apex:tab label="Details" name="AccDetails" id="tabdetails">
         <apex:detail relatedList="false" title="true"/>
      </apex:tab>
      <apex:tab label="Contacts" name="Contacts" id="tabContact">
         <apex:relatedList subject="{!account}" list="contacts" />
      </apex:tab>
      <apex:tab label="Open Activities" name="OpenActivities"
                id="tabOpenAct">
         <apex:relatedList subject="{!account}"
                           list="OpenActivities" />
      </apex:tab>
      <apex:tab label="Activity History"
                name="ActivityHistory" id="tabActiHis">
         <apex:relatedList subject="{!account}"
                           list="ActivityHistories" />
       </apex:tab>
       <apex:tab label="Contracts"
                name="Contract" id="tabContract">
         <apex:relatedList subject="{!account}"
                           list="Contracts" />
      </apex:tab>
      <apex:tab label="Travel History" name="Travel"
     id="tabTravHis">
         <apex:relatedList subject="{!account}"
                           list="Travels__r" />
      </apex:tab>
      <apex:tab label="Opportunities" name="Opportunities"
                id="tabOpp">
         <apex:relatedList subject="{!account}"
                           list="opportunities" />
      </apex:tab>
       <apex:tab label="Cases" name="Case"
       id="tabCases">
         <apex:relatedList subject="{!account}"
                           list="Cases" />
                           </apex:tab>
     <apex:tab label="Underwriting & Service" name="Underwriting_Service"
     id="tabUndeSer">
         <apex:relatedList subject="{!account}"
                           list="Services__r" />
  </apex:tab>
  <apex:tab label="GAI Opportunities" name="Opportunities_GAI"
       id="tabOppoGAI">
         <apex:relatedList subject="{!account}"
                           list="Gaiopps__r" />
      </apex:tab>
   </apex:tabPanel>
</apex:page>
Addtionally here is the screen shot from my Relataed To Account field:

I am trying to create a Tabbed Account page and keeping getting the following Error:
Activity History' is not a valid child relationship name for entity Account 
I have tried several of the recommendations given but nothing is working. Can you please review my code:
<apex:page standardController="Account" showHeader="true"
      tabStyle="account" >
   <style>
      .activeTab {background-color: #236FBD; color:white;
         background-image:none}
      .inactiveTab { background-color: lightgrey; color:black;
         background-image:none}
   </style>
   <apex:tabPanel switchType="client" selectedTab="tabdetails"
                  id="AccountTabPanel" tabClass="activeTab"
                  inactiveTabClass="inactiveTab">  
      <apex:tab label="Details" name="AccDetails" id="tabdetails">
         <apex:detail relatedList="false" title="true"/>
      </apex:tab>
      <apex:tab label="Contacts" name="Contacts" id="tabContact">
         <apex:relatedList subject="{!account}" list="contacts" />
      </apex:tab>
      <apex:tab label="Open Activities" name="OpenActivities"
                id="tabOpenAct">
         <apex:relatedList subject="{!account}"
                           list="OpenActivities" />
      </apex:tab>
      <apex:tab label="Activity History"
                name="ActivityHistory" id="tabActiHis">
         <apex:relatedList subject="{!account}"
                           list="Activity History" />
       </apex:tab>
       <apex:tab label="Contracts"
                name="Contract" id="tabContract">
         <apex:relatedList subject="{!account}"
                           list="Contracts" />
      </apex:tab>
      <apex:tab label="Opportunities" name="Opportunities"
                id="tabOpp">
         <apex:relatedList subject="{!account}"
                           list="opportunities" />
      </apex:tab>
       <apex:tab label="Cases" name="Case" 
       id="tabCases">
         <apex:relatedList subject="{!account}"
                           list="Cases" /> 
     </apex:tab> 
     <apex:tab label="Underwriting & Service" name="Underwriting_Service" 
     id="tabUndeSer">
         <apex:relatedList subject="{!account}"
                           list="Underwriting_Service" />
       </apex:tab>
       <apex:tab label="Notes and Attachments" name="NotesAndAttachments" 
       id="tabNoteAtt">
         <apex:relatedList subject="{!account}"
                           list="CombinedAttachments" />
      </apex:tab>
   </apex:tabPanel>
</apex:page>
Thank You - Julie

I need a formula that will calculate the difference between Projected Fees and Actual pro-rated Fees but does not include our first year accounts. I have tried the following: 
IF (First_Year_Account__c) = "false"
     Pro_Rata_Agreement_Fee__c  -  Projected_FY_Fee__c

 I get the following message: Error: Syntax error. Extra Pro_Rata_Agreement_Fee__c

 The First Year Account field is a check box that is only checked during the first annual term of the account.
Any help would be appreciate.
Julie J