TurboTax cannot access your 2020 online tax return.  You can download a PDF of the return yourself.  The 2020 AGI on the federal tax return, Form 1040, is on Line 11.  Or you can get a free transcript from the IRS for the 2020 tax return which will have the 2020 AGI entered in the transcript.

To access your prior year online tax returns sign onto the TurboTax website with the User ID you used to create the account - https://myturbotax.intuit.com/

Scroll down to the section Your tax returns & documents.  Click on the Year and Click on Download/print return (PDF)

If you used the desktop CD/Download editions installed on your computer, the only copy of your tax data file and any PDF's will be on the computer where the return was created.  TurboTax does not store online any returns completed using the desktop editions.

Or go to this IRS website for free federal tax return transcripts - https://www.irs.gov/individuals/get-transcript

for e-File, IRS always goes by what you originally filed,

tax season 2020 was a special situation (COVID-19), and also impacting 2021 tax season.

your AGI may not be on record.

in that case, IRS says use AGI = 0.

@haleyrockel