I am trying to setup a dynamic pageblock action on a custom case record type. It is supposed to visualize a pageblock section when the "Billing" value on the "Case Reason" picklist is selected but I get a "Error: Invalid field name for SObject Case" error when trying to quicksave it.

The idea is that when in a custom record type, (a case for account administration) you select "Billing" from a "Case reason" picklist, that a pageblock section appears with some fields that are required to be filled.

Below is the partial code I am creating, but still I have not completed it all. So if you have any ideas  to help me complete it Id be so thankful.

<apex:page standardController="Case" sidebar="False">
 <apex:sectionHeader title="Billing Training" subtitle="{!Case.name}"/>
<apex:form>
<apex:pageBlock title="Billing Training Details" id="BillingCase_dyn_pgblock" mode="edit">
    <apex:pagemessages>
        <apex:pageBlockButtons>
            <apex:CommandButton value="Save" action="{!save}"/>
            <apex:CommandButton value="Cancel" action="{!cancel}"/>
        </apex:pageBlockButtons>
        <apex:actionRegion>
            <apex:pageBlockSection title="Case Details"></apex:pageBlockSection>
        </apex:actionRegion>
    </apex:pagemessages> 
</apex:pageBlock>
</apex:form>
</apex:page>