I filed 2020 a tax return for my son (who was 17 at the time) by April 2021 and it was processed in May 2021. The correct AGI from the 2020 1040 Turbotax file transferred to the 2021 tax return. I don't think the IRS uses the PIN anymore, so when filing, I typed in 5 digits. On the first try, the tax return was rejected. I thought it was due to the PIN, but I saw someone else posted they resolved the error by putting in 0 for the AGI and that worked for me. Maybe on occasion, the IRS doesn't have the prior year's AGI in their system - maybe if 2021 was only the second year the person filed a return. Having to enter 0 in the AGI is not necessarily because the prior year was filed late, it could just be IRS not keeping up.
