TurboTax asks you what type of health-related accounts you have, although this screen sometimes has to be sought out.
First, did you receive one or more 1099-SAs, reporting on money spent out of your MSA? If you didn't, then don't worry about reporting your MSA. I am a little concerned about your insurance not being an HDHP, because that is normally part of the process for having a Medicare Advantage MSA - see this screen.
In your case, go to Medical->1099-SA, MSA, HSA. On the next screen, check off that you have an MSA. Then say that you spent money out of your MSA, so you can enter your 1099-SA(s). Then continue on, noting that you are on Medicare and that you did not contribute anything to the MSA.
Please have a look at this CMS screen (Centers for Medicare & Medicaid Services) which is "Qs and As on Tax Reporting for ...".
Like I said, you are just trying to enter any 1099-SA(s) received; otherwise, just ignore it.
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