You need to go back in to your Illinois return and fix your AGI from 2020. Your return was rejected because Illinois says the 2020 AGI that you entered did not match the AGI on your 2020 return. It will be on line 9 of your IL-1040 from 2020. If the AGI matches then you need to contact the Illinois Department of Revenue.
According to Illinois, IL your Tax Return needs a correction. see if there is any more information in the rejection notice.
depending on what you're using to e-file Il - il pin , driver's license number , other IL id
make sure all the info is correct
How do I make the change on my AGI on my state return. I cannot figure out how to make the change
The prior year AGI is normally an identification step in the e-file process. Go back to the filing interview and you will be asked to enter your prior year AGI or as @Mike9241 mentions any other ID methods you might have used.