On the Help menu there is an option to add a Payroll ID code or when you open the Payroll module (Paycheques) it should automatically give you the option to add the code.

If you have done that and the program is still indicating it is not valid, then there is likely a code/version mismatch.  You must be on the current 2021 version to have active Payroll at this point as 2020 is now obsolete.  Therefore I don't think Sage would be given out new codes for the 2020 version.

Under Help, About Sage 50...., Support Info, does it indicate Payroll is Active?

Stephanie: Go to Help and Update Service Plan. Be sure your account number is correct and then click on check Service Plan. On occasion when a new plan is purchased it takes a couple of hours for it to get registered in the Sage file. Try this to see if the results are there.