I think people on this thread are getting confused (or maybe I'm confused, I suppose).
Medicare is Medicare. Not all medical facilities take it. The ones that don't, are likely not "approved" by Medicare or simply don't want to do work at approved Medicare rates.
Medicare Supplemental or Advantage, is an entirely other type of medical coverage. Again, medical facilities can decide what Insurances they'll accept or not accept.
Facilities can take Medicare, but not take your Supplemental or Advantage Plan.
Does your provider accept Medicare as full payment? | Medicare