Until you have received the product or service, there is no actual debt. They don’t have to serve you if you won’t pay cash, don’t have to let you walk out of the store with merchandise, etc.

Arguably at a sit-down restaurant where you’re ordering and being served before you pay, then there is a debt and they may have to accept cash payment

But, and I’m not a lawyer so take this with a big grain of salt, I think it’s still their right to only accept payment in whatever form they want to, you’re effectively agreeing to their rules when you choose to patronize their business, and the thing about it being legal tender only really comes into play when they get the courts involved to collect on that debt- if you refuse to pay with electronic payments, gold coins, crypto, shiny beads, or whatever they insist on, and they take you to court over it, then they have to accept cash to settle that debt once that’s all sorted out.