Federally, assuming you are speaking of the US, a business refusing to take cash for goods/services is not illegal, as you are not paying a debt:

https://www.federalreserve.gov/faqs/currency_12772.htm

Business owners should note that state and local laws can and do vary on this topic!

https://blogs.findlaw.com/free_enterprise/2019/08/is-it-lega...