If you need to create a payment for an employee who left in a previous tax year but find the option is greyed out, you should set the employee up 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 should enable the payment option that was previously unavailable. 

This option can be found under the Payroll tab > More > add payment after leaving option.