You are asked if you had marketplace health insurance from healthcare.gov because if you did, you have a form 1095A that you are required to enter. If you had Medicare, then when it asks if you have a 1095A you say no and keep going.
If you are getting Social Security benefits, your Medicare is deducted from your SS. When you enter the SSA1099 for your Social Security, the amount paid for Medicare flows automatically to the medical expense section of the software, so do not enter it again.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**