I had to a 1040X on my previous years return. Despite me going to H&R Block for professional help, there was still apparently an error. Many months later, I received a letter from the IRS stating I owed. Called the number on the letter, and was told the IRS actually made a mistake and to fax some information in. I did that, and I'm waiting. I was told it would take 4-6 weeks to reopen and review the case. The problem is I cannot file my current years return without last years AGI and in 4-6 weeks, the already extended filling deadline will have passed.
When you electronically sign your 2020 tax return for e-filing, you use the AGI on your original 2019 tax return. The 1040-X and the IRS changes don't matter.
Did the IRS actually accept your return? If not, your AGI is probably the original AGI. Try calling the IRS number on the notice,. They should be able to provide you with your AGI. Or get an IRS transcript online.
A transcript shows most line items including the adjusted gross income (AGI) from your original Form 1040. It doesn't show changes made after you filed your original return. This transcript is only available for the current tax year and returns processed during the prior three years.
You can obtain an IRS transcript at Welcome to Get Transcript.
As a last resort, you can mail your return. Send it by certified mail with return receipt so you have proof the IRS received it.