Unfortunately, the IRS was backlogged in 2020 due to covid, which caused allot of 2020 returns to be processed late. That caused allot of returns to be rejected with the actual 2020 AGI. When returns are processed late, a Zero for the 2020 AGI works because in the IRS system is shows it was processed late.
TurboTax does not know any information about the processing of returns, so they would not know your return was processed late, so they cannot suggest to enter a Zero for the AGI. The IRS or State does not update TurboTax once your electronic returns are accepted.
