you don't file form 8879 in 2021. that's only for independent paid preparers who e-file a client's return. it gives the preparer authoriztion to e-file.
when Turbotax asks for your prior AGI use the amount from part 1 line 1 of the 8879 which should match the 2020 1040 return line 11.
if you filed after 10/15/2021 use zero instead.
if that doesn't work go back and see if you can indicate you didn't file last year
if nothing works you'll have to mail in your return.
