@mitty2day
This example from page 4 of Illinois Publication 130 would seem to describe your situation precisely:
"Mary is an Missouri resident who earned $60,000 in wages from her employer for the tax year. During the
year, she performed services for her employer for 35 days in Illinois out of 250 total working days for the year. Accordingly, 14 percent (35 working days divided by 250) of Mary’s wages, or $8,400, was paid in Illinois and is subject to Illinois income tax withholding."
https://www2.illinois.gov/rev/research/publications/pubs/Documents/pub-130.pdf
Work income paid in Illinois is taxable by Illinois.
In my opinion you must file a non-resident Illinois tax return in addition to your home state Missouri tax return. (If your main or primary home - your domicile in tax terminology - is in MO, all your income is taxable by MO.) You'll be able to claim a credit on your MO return for the taxes paid to IL, so you won't be double-taxed.
In TurboTax, be sure to complete the non-resident state return before the home state return, so that the program can calculate and apply the credit correctly.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
