ENTER 0 FOR AGI. The IRS took a long time last year to process 2020 returns so the AGI didn't get entered in time. Especially if you mailed it. That works for most people. See,
How to correct the AGI in the Online version
If you can’t get it to efile you will have to print and mail it.
Be sure to attach copies of your W2s and any 1099s that have withholding on them. You have to mail federal and state in separate envelopes because they go to different places. Get a tracking number from the post office when you mail them for proof of filing.
I have been having the same issue. I did an online search for others having the same problem and found comments saying that the IRS does not recognize your prior year AGI if they have not yet processed your return. Others have suggested putting in zero for prior year AGI works.
I resent mine earlier today using 0 for AGI and so far it has not kicked back. Fingers crossed.