I had this issue too. I entered the AGI exactly as I had entered it on my 2019 tax return and my federal form was rejected. I even logged into the IRS website and checked my AGI matched with my 2019 tax return receipt.
I tried resubmitting my return via e-file, this time I changed my AGI to $0, based on the feedback from this article and it went through successfully. I got an acceptance email within minutes of resubmitting it. Maybe since I had filed my tax return by mail last year, the number didn't propagate to all of the IRS systems, I'm really not sure why - but entering zero fixed the problem. Thanks to all that contributed.
P.S. There is no part of this process that is a scam - wish someone could fix the title of this help.
