I don't see a reason why you can't, since you had a license for the 2020 software. You should be able to do the amendment.

If you dont have a current Intuit FastPath license, you will not be able to print or Efile any new returns.....you may however still be able to print a 1040X for a client that you did file a 2020 return for (theyre not a new client)...have you tried?

Yes I can do the amendment, but I just figured out I can’t efile it. 

I just tried to efile the amendment but can’t because no FastPath license. Btw, you can print new returns, you just can’t efile them.