I just upgraded to Premier Plus Edition 2021 and had the same problem with the print queue not clearing no matter how many times I printed the invoices.  I then followed all the steps in this thread but still had the same problem.  I ran the "verify data" and no errors showed but went ahead and ran "rebuild data".  Went back and printed the invoices again and this time it cleared it!!  Not sure if anyone who had this problem tried that but it worked for me.

Good luck!