We mailed a 2019 return (married filing jointly) a bit late - a few days after the October 15 extension deadline. It now DOES show in our online IRS account with an AGI for 2019, but this number was rejected when we tried to efile for 2020.
Entering $0 for *both* me and my spouse worked. TurboTax made me check a bunch of boxes that don't make sense when you're trying to enter $0 for the spouse, but I decided to try anyway, and the return was accepted.
Apparently whatever system the IRS uses to check the AGI number is NOT in sync with the information you can see in the online account view, and in our case I believe it was because our 2019 return was not processed by the IRS until about a month ago. So if your 2019 return was late due to your own filing date or the IRS taking forever to process paper returns for 2019, I would suggest trying $0 before you give up on e-file.
One of the reasons we were late filing for 2019 was the same problem where the efile wouldn't accept the true AGI number - at the time I didn't figure out that we could try $0 (TurboTax wasn't as helpful here as I would have liked). Hopefully next year when I e-file it will finally accept the true AGI number for 2021.
