If your 2020 return was accepted after April 15 last year the IRS doesn't have the AGI on file in the right server/database to lookup to verify you. So you have to enter zero AGI to efile 2021. That happened to a lot of people.
What error message do you get when you try to efile 2021? Why is it rejecting? It could be another reason. Like someone you don't know made a mistake and accidentally entered a wrong ssn on their return and entered your ssn by mistake.
When did you file 2020?
