I have a controller extension for a custom object where I am trying to insert a record according to input from selectCheckbox. I am getting 'URL No Longer Exists'  and am unable to debug this or find out what is going wrong. 
Markup:

<apex:page standardcontroller="Equipment_Type__c" extensions="addMakesController1">
  <apex:pageBlock id="pageBlock1">    
        <apex:pageBlockTable value="{!usedMakes}" var="ma" style="width:20px">           
            <apex:column headerValue="Action">
                <apex:form >
                <apex:commandlink action="{!delMake}">Del</apex:commandlink>
                </apex:form>
            </apex:column>
            <apex:column value="{!ma.name}" />
        </apex:pageBlockTable>
        </apex:pageBlock>
        <apex:pageBlock id="pageBlock2">
        <apex:form >
            <apex:selectCheckboxes value="{!selectedMakes}" style="float:left"  layout="PageDirection" >
                <apex:selectOptions value="{!validMakes}" />
            </apex:selectCheckboxes><br/>
            
        </apex:form>
  </apex:pageBlock>
      <apex:form >
      <apex:commandButton action="{!addMakes}"  value="Add selected Makes" rerender="pageBlock1,pageBlock2" status="updateStatus"/>
                <apex:actionStatus startText="Please wait..." stopText="Records updated successfully" id="updateStatus"  />
      </apex:form>
</apex:page>

 Controller:

public with sharing class addMakesController1 {
    private Equipment_Type__c type;
    public addMakesController1(ApexPages.StandardController controller) {
        this.type = (Equipment_Type__c)controller.getRecord();
        
        list<Equipment_Type__c> type1=[select id,Functional_Group__r.Id from Equipment_Type__c where id=:this.type.id limit 1];
        if(type1.size()!=0)
        this.type = type1[0];
    }
    public list<Make__c> allMakes=new list<Make__c>();
    public list<SelectOption> temp=new list<SelectOption>();
    public list<Make__c> displayMakes=new list<Make__c>();       
    public list<Make__c> getUsedMakes(){
        allMakes=[select name from Make__c where Functional_Group__c=:type.Functional_Group__r.Id];
        list<Make__c> usedMakes=new list<Make__c>();
        for(Make__c m:allMakes){
            list<Type_Make__c> tmtemp=new list<Type_Make__c>();
            tmtemp=[select id from Type_Make__c where Type__c=:type.id AND Make__c=:m.id];
            if(tmtemp.size()!=0){
                usedMakes.add(m);
            }
        }
        return usedMakes;
    }         
    public list<string> selectedMakes{get;set;}
    public list<SelectOption> getValidMakes(){
        allMakes=[select name from Make__c where Functional_Group__c=:type.Functional_Group__r.Id];
        list<SelectOption> validMakes=new list<SelectOption>();
        for(Make__c m:allMakes){
            list<Type_Make__c> tmtemp=new list<Type_Make__c>();
            tmtemp=[select id from Type_Make__c where Type__c=:type.id AND Make__c=:m.id];
            if(tmtemp.size()==0){
                validMakes.add(new SelectOption(m.name,m.name));
            }
           
        }    
        if(validMakes.size()!=0){
            return validMakes;
        }
        else{
            return null;
        }
    }
    public void delMake(){}
    public void addMakes(){
            list<Type_Make__c> tmList=new list<Type_Make__c>();
            for(String m:selectedMakes){
                tmList.add(new Type_Make__c(Type__c=type.id,Make__c=[select id,name from Make__c where name=:m].id));
                 
           if(tmList.size()!=0)insert tmList;
        }
    
}