https://en.wikipedia.org/wiki/Legal_tender
It doesn't prevent you from accepting other means of exchange (though in some countries it does), but it does force you to accept it, even if you don't like it.
Tim DeChristopher spent 21 months in prison despite how he "had raised sufficient funds for an initial payment to the BLM (which the BLM refused to accept)."
https://en.wikipedia.org/wiki/Tim_DeChristopher#Trial
(I bring this up because I'd always wondered about how why that case fell out that way, and apparently it can't, per the common meaning of the terms, so my understanding of legal tender seems correct?)