Hi cattleking,
If you are using third party payroll software, please contact your payroll software provider for assistance with printing the P45.
If you are using HMRC Basic PAYE Tools, you will be able to print this only after submitting the Full Payment Submission (FPS) that contains the leaving date for the employee.
To do this you will need to select manage employees, click on the name of the employee concerned and select leaver summary.
Here you will see the submission status and an option to print the P45 if it has been submitted.
If you have any problems with this, then please contact HMRC Online services.
Technical support with HMRC online services
Thank you.