it is entirely possible that someone filed using your SSN. a transcript or even record that a 2020 return may not show up for 2-3 weeks after it is fully processed.
you can try 0 as AGI but an AGI problem is inconsistent with the rejection message you got which points to an SSN and not an AGI issue. perhaps wait till tomorrow and try to e-file. continued rejection means you'll have to file by mail. you may want to enclose an identity theft affidavit form 14039
