Where can I find AGI for 2020 ?

On my form from 2020, the AGI is on line 11 of Form 1040-SR.

I'm a senior but it must be similar to the standard 1040.

See this TurboTax FAQ on how to find your AGI

https://ttlc.intuit.com/community/agi/help/how-do-i-find-last-year-s-agi/00/25947?search-action-id=235856632119&search-result-uid=25947

You can get free 2020 tax transcript from the IRS

https://www.irs.gov/individuals/get-transcript

If you filed late last year there is a good chance that the 2020 AGI never was entered into the e-file database. Try using 0 (zero) for the AGI.

If you filed an amended return use the AGI on the first filed tax return.

If it continues to reject then the AGI in the IRS database might be in error. Mail filing is the only option.