I entered ALL items from Boxes 5 & 6 into Turbo Tax Basic and under Health Information, I answered that all filers on this return were covered by health insurance for all year of 2017. When I print Form 1040A, line 38 is not checked.
Yes, line 38 of the 1040A should be blank if you are on Medicare.
That line is for the Affordable Care Act (ACA) for an additional tax on your return if you had ACA and did not pay enough premiums to the Health Care Exchange.
So if you are covered under Medicare you are not covered by ACA and have no additional taxes to be paid.