This happens when a PAYE worker hasn't been P45'd from their previous employment and you are trying to bring them in for a new Employment - however this worker record is attempting to update the existing record, which already has data against it, so is erroring.
You have two options:
Leave the existing PAYE worker and then start the new PAY worker next Period or
Leave the existing PAYE worker and create a new PAYE record for the new worker.
📌 Note: The second option would be more complicated, as your P&B records are linked to the front office records so the worker would need everything setting up fro scratch.