We understand you are having an experience with entering your AGI. Please follow the instructions in this link: AGI Assistance . If this does not resolve your issue, what is the exact message you are getting? 

Having the exact same issue.  My elderly dad passed away in 2019, and I have been filing taxes for both my mom and dad for a number of years.  A combination of issues including their filing status (married/joint) in 2019, a stimulus payment that was based on 2020 filing status, and the fact that they were due a refund that I was applying to 2020 taxes created a scenario where I simply had to file by paper for 2019.

Shifting to 2020, I am trying to do an electronic return, but the AGI is not recognized.  I just got off the phone with the IRS and confirmed that they were still "working on" paper filed returns (including this one for 2019.  Thus 2019 AGI was not yet on their computers.  The guidance was to:  1) Try "0" in the AGI for 2019, 2) Try the 2018 AGI for 2019 as it was the last AGI recorded, and then 3) File by paper if 1) and 2) don't work.