You are correct in that your income needs to be allocated between IL and MO to reflect that 15% of the wages you earn are for services performed within MO. The remaining 85% of your wages are for services performed in IL. The MO Department of Revenue supports this type of allocation in that they have stated the following:
"If you perform services partly within and partly out of the state, only the wages you are paid for the services performed within Missouri are subject to Missouri income tax. The Form(s) W-2 issued to you from your employer(s) should indicate the state in which the wages were paid, along with the wages you earned in that state. You must allocate your Missouri source income on Form MO-N RI and complete Form MO-1040."
Regarding the process to correct the allocation, review your entries in the Personal Info section of TurboTax. Make sure your residency is IL but that you have also indicated that you earned income in another state, e.g., MO. As you went through the MO section, you should have seen a page entitled "Remote Work," and that is where you will enter your remote income amount. For example, the instructions on this page provide the following with regard to your remote work:
"If you performed remote work for a Missouri employer outside of Missouri during 2020, the portion of income earned while working outside Missouri is not taxable to Missouri. Amounts entered as remote work income will be subtracted from Missouri sourced wages transferred from your federal return."
Review the MO section again, and follow-up with additional information regarding whether you have seen the "Remote Work" page and what amount you entered on that page assuming you were able to find it.
Below is a link to the MO Department of Revenue which explains how, among other things, remote work is handled.
MO Department of Revenue
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