There could be a few reasons you may not qualify for the Other State Tax Credit (OSTC) in Illinois. The OSTC is a tax credit that taxpayers can claim on their Illinois tax return if they have paid income tax to another state for income also taxed by Illinois. Here are some reasons you may not qualify: 1. Nonresidents: If you are a nonresident of Illinois, you are not eligible to claim the OSTC in Illinois. 2. Proper allocation: To claim the OSTC, the income must be properly allocated between Illinois and the other state. If the income isn’t correctly attributed, you may not qualify for the credit. 3. Double taxation: The OSTC aims to prevent double taxation of the same income. If the income in question hasn't been taxed by both Illinois and another state, you might not qualify for the OSTC. 4. Reciprocal agreements: Illinois has reciprocal agreements with some states, which allow Illinois residents to work across state lines without paying tax to the other state. If Illinois has a reciprocal agreement with the other state in question, the OSTC may not be applicable. These are general guidelines, and the specifics of your situation may vary. It's important to review your tax documents and the Illinois tax laws to determine if you qualify for the OSTC.