is my insurance subscriber no. my medicare number?

[xmasbaby0]

Where are you trying to enter a Medicare subscriber number?   That does not get entered on a tax return.   You are asked if you have a 1095A for marketplace health insurance.  If you have Medicare, you do not have a 1095A.  You need to just say NO to the question about a 1095A and then say you had health insurance all year.   You do not enter anything about your Medicare number.