Another line says to enter "0" because they probably haven't processed your last-year's return yet. It's a mess right now. They don't even have a payment I sent in June of last year on their records, but they cashed the check 3 days after I mailed it. Now this year, I'm getting the same message AGI doesn't match. Turbo Tax should be aware of this and put the information in the program instead of telling you to go hunt down your AGI (which THEY filled in correctly for you from last year). It's a frustrating mess that I think the feds are responsible for, but Turbo Tax is not helping by not reacting to the changes. You may have to mail it in, but then likely you won't see anything for like 8-10 months. I don't know, I'm just trying to work with the lack of information I have as well.
Follow up: I entered "0" AGI and it was accepted. Their slowness on the 2019 returns requires you to put in the wrong amount (0), to be considered right by the government. Oh the hoops we go through. TurboTax should have something on their "AGI" entry page warning people. This is real. My guess is that most returns will probably be rejected if filed around the extended June/July timeframe of last year.