When you attended school in MO and worked in MO, but were a student who was "temporarily absent" from your family home in IL, (and were being claimed as a dependent of your IL parents) you were supposed to file a non-resident return for MO and a resident return for IL. Sounds like you skipped filing in IL and now IL wants its share. If the return you filed for MO was not a NON-resident return, you need to amend that one and file as NON-resident and then prepare a 2021 IL return. The NON-resident one has to be done first.
@Mike9241 Your thoughts?
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