Hi jemmab1734,
Thanks for your post. 😊
In QuickBooks Advanced payroll, it's correct to record the employee as a leaver before running them on their final payroll. This way, HMRC will be notified of the leaver in the FPS and a P45 will be generated within QuickBooks.
To record an employee as a leaver, you would select the option to 'Terminate Employee' and enter a termination date, which should fall within the pay period dates of your next pay run.
As you can only submit one FPS per pay period, to resolve this you should roll-back and re-run this pay run which will then allow you to re-submit as a correction to an earlier submission.
To rollback the payroll, simply go to Pay Runs > Select the relevant pay-period listed > Select Pay Run Actions > Unlock > Delete.
You will then be able to re-run this period by selecting 'New Pay Run'. Please get back to us below if you have any questions on this!