I am confused on the penalty for health insurance. I had coverage with Medicare all of 2018 so why am I being penalized?

If you had Medicare, did you select that "Yes" you had coverage and "No" it was not from the marketplace? 

Please guide me through this 

If you had Medicare for all of 2018, when you get to the health insurance screens you only have to answer two questions:

Did you have health insurance coverage for 2018       Click that you had insurance

Next screen--"was anyone enrolled in one of these less common plans....."  Click NO

Done.