According to the forms availability table for Michigan, Form 4884 is available. Are you using TurboTax Online or TurboTax Desktop (CD/Download)?
If you are using TurboTax Desktop, be sure your software is updated. To do that, see the instructions I'm linking below.
Mine is still getting flagged for form unavailable and won't file MI state taxes. My federal and Indiana were both accepted. And my software is up to date.
@tomdawn1995 The state forms availability table is showing that your form is available.
Forms availability TurboTax MI Windows
Since you're still being flagged that the form is unavailable, would you mind doing a manual update of your software, please? I'm linking instructions below.
Manually update TurboTax for Windows software
Hopefully, this will resolve your issue. If it doesn't, please contact TurboTax Customer Support during business hours using the method I'm linking below.