For instructions fixing your rejected Illinois state return please see the FAQ below.
Note: After trying the below steps if you get the same rejection a second time and you see no mistakes on your return, select the file by mail option to paper-file your return. See: How do I print and mail my return in TurboTax Online?
See TurboTax FAQ (Scroll down to the solution section): https://ttlc.intuit.com/replies/3686230
