I entered the correct AGI, why is my returned being rejected?

Does the rejection error code say it's being rejected due to the 2019 AGI?

Be sure you are using the AGI from the original 2019 return, not an amended one.  The figure is on your 2019 Form 1040, Line 8b.

If your return was filed real late in the year, such as after mid-November 2020, you can use an AGI of 0 (zero).  Or if your 2019 was hung up in processing at the IRS for many months and wasn't finished processing until November 2020 or later, you can try an AGI of 0.

If it's a joint return, both spouses use the same total 2019 AGI.  Do not divide or otherwise allocate it.

If you qualify to open an IRS account, you can download a free 2019 tax return transcript to what the IRS is showing for your 2019 AGI.

https://www.irs.gov/individuals/get-transcript

Whatever you do, before giving up, try an AGI of 0.