The IRS sent you this request because they received information from the federal marketplace or a state marketplace that you had acquired insurance under the Affordable Care Act (aka Obamacare).
Naturally, sometimes mistakes happen in this process, as someone under Medicare should not have to do this.
#1
Did you receive a 1095-B from Medicare for last year? If so, reply to the IRS letter stating that you were under Medicare (attach a copy of the 1095-B) and did not have any insurance under the Affordable Care Act that would require an 8962 (if that's true).
#2
Any chance that you were named on someone else's return as being a part of their Obamacare policy (say, by a child)? That would be enough to trigger the IRS alert. (and ask your relative to take you off the Obamacare policy).
The purpose of the 8962 is to determine how much Premium Tax Credit (PTC) you should have received in 2017. The PTC is used to pay down your insurance premiums (that's why it's the "affordable" care act). I doubt that you received any PTC, and the person who placed you on the policy (if someone did) owes you a completed 8962 in which none of the credit is allocated to you. You would send this 8962 to the IRS and that should be the end of it.
In this case (if you were unknowingly put on someone else's policy), tell your relative to go to a competent tax professional (Enrolled Agent or CPA) and ask them to create 8962 forms for each person on the policy.
