If you need to create a payment for an employee who left in a previous tax year but the option is greyed out, you'll need to re-add the employee in the current year's employer file.

Ensure that the employee details match those from the original file, including the same start date, leave date, and Payroll ID.

This will enable the payment option that was previously unavailable.

You will find this under the Payroll tab > More > add payment after leaving option.