Ah, so a referendum is a direct vote by the population on a given issue - for example a lot of states have passed recreational marijuana referendums, in my opinion at least because a lot of lawmakers didn’t want to to be seen as supporting it, but you can’t get blamed if the public approved it directly.
I’m not aware of any state level referendums on universal healthcare (which doesn’t mean that there haven’t been any) and there isn’t a national level referendum. (Although in googling this to confirm that I found an interesting article about implementing a national referendum)
With the Medicare for All Act it’s been introduced as a bill, but as I understand the process it first needs to be reviewed by a committee and voted out of that committee before the senate or house can consider it to possibly hold a vote. Then it needs to do the same thing in the other chamber of congress. So you can imagine that’s a lot more convoluted process than a referendum, and while voters may ask their representative to pass it, plenty of opportunities for legislators to say, “oops, some technicality or person who’s not me has stalled the process.”