I just resolved the same problem using the $0 for AGI, even though my AGI number matched the one in the IRS's transcript for 2020, but it took two tries. After I entered 0 and resubmitted it came back with a rejection of the spouse's AGI. If you are married filing jointly then you need to enter 0 for both people, not just the primary filer and that isn't obvious (at least in the CD/download version.) Also make sure that your PIN numbers match the ones that were used last year if you e-filed. The error is for either not matching their records (although the $0 fix is matching some secret record that we can't know about.)
Given the timing of your message there is a chance that you haven't actually mailed them yet.
Good luck!
