The employee must have a record in Job Data with the payroll number you are trying to pay. As long as the Job Data record has a row with the payroll number you are trying to pay on, it will let you process on the PCS.
Check the Job Data tab and see if the payroll number is on the records. If the payroll number you are trying to pay on is not there, please consult with your department HR office to see if the payroll number can be updated for the earnings dates you are trying to pay.