See this TurboTax FAQ on how to find your AGI
https://ttlc.intuit.com/replies/3301618
You can get free 2019 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 2019 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.
Also see: https://www.taxpolicycenter.org/taxvox/are-irs-security-tools-blocking-millions-people-filing-electronically
https://www.forbes.com/sites/howardgleckman/2021/04/26/are-irs-security-tools-blocking-millions-of-people-from-filing-electronically/?sh=529e1d891a76
Since the IRS is rejecting the correct 2019 AGI there is a good possibility that the IRS does not have your 2019 AGI in their database. Try entering a 0 (zero) for the 2019 AGI and e-file the 2020 tax return again.