According to the Michigan tax instructions, line 18 should show "only income tax paid to another government unit(s) on income earned while you were a Michigan resident and taxed by Michigan."
https://www.michigan.gov/taxes/-/media/Project/Websites/taxes/Forms/2021-IIT-Forms/Book-1040-Instruc...
Did you (the Michigan resident) have any income that was taxed by another state? If not, line 18 should be zero.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
