I have 2 custom Object.

A__c <- Main Object
B__c <- Related Object with many records.

Primaryrecord__c <- Checkbox in B__c object
Aname__c <- is the lookup in the A__c object to B__c. 

I'm current trying to create Primary Record that will produce and error when another primary record checkbox is selected.  I'm trying to copy the code below by replacing the objects and line commands but doesn't seem to work well.   Are there any Apex trigger that can help me with this?

this code is functioning well on my Accounts and Contacts object.  I just need it for 2 different objects

Code Below

Trigger PrimaryContact on Contact (Before Insert, Before Update, 
                                   After Insert, After Update, After Delete, After UnDelete) {
    
     List<Id> actIds = New List<Id>();
     List<Contact> comingCons = New List<Contact>();
     List<Id> conIds = New List<Id>();
     
     If(Trigger.IsBefore && (Trigger.IsInsert || Trigger.IsUpdate))
     {
         For(Contact Con : Trigger.New)
         {
             If(Con.Primary_Contact__c == TRUE)
             {
                actIds.add(Con.AccountId);
                conIds.add(Con.Id);
                comingCons.add(Con);
             }
             
         }
     }
     
     List<Account> allRelatedAccounts = [Select Id, (Select Id, Primary_Contact__c 
                                                     FROM Contacts WHERE Primary_Contact__c = TRUE AND Id !=: conIds)
                                                        FROM Account WHERE Id =: actIds];
     
     
     For(Contact EveryCon : comingCons)
     {
         For(Account EveryAccount : allRelatedAccounts){
             If(EveryCon.AccountId == EveryAccount.Id && EveryAccount.Contacts.size() > 0){
                 EveryCon.addError('There is already a primary contact for this account');
             }
         }
     }                                  
                                       
     
                                   
    List<Id> accountIds = New List<Id>();
    
    If(Trigger.IsInsert || Trigger.IsUpdate || Trigger.IsUnDelete){
        For(Contact Con : Trigger.New){
            If(Con.AccountId != NULL){
                accountIds.add(Con.AccountId);
            }
        }
    }
    If(Trigger.IsDelete){
        For(Contact Con : Trigger.Old){
            If(Con.AccountId != NULL){
                accountIds.add(Con.AccountId);
            }
        }
    }
    
    List<Account> actFinalListToUpdte = New List<Account>();
    
    
    For(Account act : [Select ID, Contact2__c,
                            (Select Id, FirstName, LastName,
                                    Primary_Contact__c FROM Contacts WHERE Primary_Contact__c = TRUE LIMIT 1)
                                FROM Account WHERE Id =: accountIds])
    {
        If(act.Contacts.size() > 0)
        {
            act.Contact2__c = act.Contacts[0].Id;
            actFinalListToUpdte.add(act);
        }
            
    }
    
    try{
        If(!actFinalListToUpdte.isEmpty()){
            update actFinalListToUpdte;
        }
    }Catch(Exception e){
        system.debug('Thrown Exception for PrimaryContact is: ' + e.getMessage());
    }
}

when I change things I get errors like...

- Error: Compile Error: Variable does not exist: AccountID at line 14 column 32
What should I change AccountID to?
- Didn't understand relationship 'B__c' in FROM part of query call. If you are attempting to use a custom relationship, be sure to append the '__r' after the custom relationship name.

Any guidance will help.  Thanks