No. If you are already receiving Medicare coverage it is against the law for someone to sell you an Affordable Care Act (ACA) Health
Insurance Marketplace plan - assuming they they know that you are receiving coverage
through Medicare.
If you are receiving Medicare Part A and Medicare Part B coverage, then instead of an ACA plan, you are eligible to get additional healthcare coverage through a
Medicare Supplement (that adds benefits to your Part A and Part B coverage) or a
Medicare Advantage plan (that implements your Part A and Part B - and may include drug coverage
(Medicare Part D) and additional or
supplemental benefits).
If you are enrolled in an Affordable Care Act (ACA) health plan and now are eligible for Medicare, you will no longer receive any premium subsidies if you qualify for "premium-free" Medicare Part A and would pay higher ACA premiums for your plan - so you can keep your plan but pay a higher premium and probably get inferior coverage as compared to Medicare plan coverage.
If you are enrolled in an Affordable Care Act (ACA) health plan and now
are eligible for Medicare but do not qualify for "premium-free" Medicare Part A, you can keep your ACA plan, and not join Medicare Part A or Part B when you are first eligible - but this may mean that you - again, have inferior coverage as compared to a Medicare Advantage plan or Medicare Supplement - and you may pay a late-enrollment penalty for your Part A and Part B (and Part D) coverage when you later sign up for Medicare - which is not recommended.