Hi Milano,

You are too too good. Thanks once agin for all your help so far. The delete logic is now perfectly working fine.

In the insert update logic there is a help required. A logic change in the error message:"There is already a Potential Contact associated to this Account. Please uncheck and try saving the Contact."

--When user creates a new contact with check box ticked, the trigger should allow to save but make the check box Unchecked for the existing contact rather than throwing the above error message.

For my learning I was trying to modify the code but not successfull. Got the error message at line34:-  relContactId.Campaign_Contact__c=false --: 

Error: Compile Error: Initial term of field expression must be a concrete SObject: Id at line 34 column 17

Below is the overall code:

trigger PotentialContactCheckbox on Contact (before insert,before update, after delete) {
  
// START UPDATE and INSERT LOGIC 

   if ( Trigger.isBefore && (Trigger.isInsert || Trigger.isUpdate) ) {
    // get set of ID for related Accounts
    Set<id> accIds = new Set<id>();
    for (Contact con: Trigger.new) {
        if ( con.AccountId != null ) {
            accIds.add(con.AccountId);
        }
    }

    // find contacts with Pot_Contact__c equals to TRUE only for related Accounts
    List<Contact> relContacts = [SELECT AccountId FROM Contact WHERE Pot_Contact__c = true AND AccountId IN :accIds];
   
    // build a map with key = AccountId and value = ContactId
    Map<Id,Id> relAccContMap = new Map<Id,Id>();
   
   
    for (Contact con: relContacts) { //WHERE Pot_Contact__c = true
        relAccContMap.put(con.AccountId, con.Id);
       
    }

    // check for correct set of Pot_Contact__c field
    for (Contact newcon: Trigger.new) {
       
        Id relContactId = relAccContMap.get(newcon.AccountId); // ID of existing Contact or null
       
           
          if ( newcon.Pot_Contact__c ) {
            if ( relContactId != null){                // && relContactId != newcon.Id ) {
               
             // newcon.Pot_Contact__c.addError('There is already a Potential Contact associated to this Account. Please uncheck and try saving the Contact.');
             
             relContactId.Pot_Contact__c=false;
  
            }
        }  else {
             if ( (relContactId == null || relContactId == newcon.Id) && newcon.AccountId != null ) {
                newcon.Pot_Contact__c.addError('There has to be one Potential Contact associated to this Account. Please check the check box and try saving the Contact.');
            }
        }
   // }  
    }
} // END UPDATE and INSERT LOGIC

// START DELETE LOGIC

if ( Trigger.isAfter && Trigger.isDelete ) {
    // get set of ID for related Accounts for OLD (deleted records) with Pot_Contact__c == true
    Set<id> aIds = new Set<id>();
    for (Contact con: Trigger.old) {
        if ( con.AccountId != null && con.Pot_Contact__c ) {
            aIds.add(con.AccountId);
        }
    }
    // find contacts for related Accounts
    List<Contact> lastContacts = [SELECT AccountId,CreatedDate FROM Contact WHERE AccountId IN :aIds];
    // build a map with key = AccountId and value = recently created Contact
    Map<Id,Contact> accContMap = new Map<Id,Contact>();
    for (Contact con: lastContacts) {
        Contact c = accContMap.get(con.AccountId);
        if ( c == null || con.CreatedDate > c.CreatedDate ) {
            accContMap.put(con.AccountId, con);
        }
    }
    // set Pot_Contact__c field equals to true for recently created Contacts
    for (Contact con: accContMap.values()) {
        con.Pot_Contact__c = true;
    }
    update accContMap.values();
}  // END DELETE LOGIC

}