Can I add a new employee to be paid this month after I have done the payroll run?

Hi youthoptions, thanks for joining us here on the Community,

It's not possible to edit a payroll once this has been completed, though you can roll this back to re-do and then resubmit to HMRC. 

So that we can guide you on the steps to rollback and resubmit, can we check if you're using the Standard or the Advanced payroll? 👩‍💻