If you are on Medicare, you can't contribute to an HSA.
The IRS says
"Qualifying for an HSA
To be an eligible individual and qualify for an HSA, you must meet the following requirements.
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You are covered under a high deductible health plan (HDHP), described later, on the first day of the month.
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You have no other health coverage except what is permitted under Other health coverage , later.
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You aren’t enrolled in Medicare.
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You can’t be claimed as a dependent on someone else's 2016 tax return."
See IRS Pub 969. - https://www.irs.gov/publications/p969/ar02.html
The last month rule actually refers to whether or not you joined an HSA in 2015 (yes, 2015) and made contributions. I am guessing that in 2015, you were also on Medicare, so you weren't eligible to make a contribution then, either.
You need to notify the HSA plan administrator and ask them to send you a "Mistaken Contribution" form, so that you can withdraw all those funds that you contributed.
