There is law that they must accept any official us currency to pay debts. This is not official us currency.
The following is according to the federal reserve. It may be different in another country, but it looks like this post is from the US.
There is no federal statute mandating that a private business, a person, or an organization must accept currency or coins as payment for goods or services. Private businesses are free to develop their own policies on whether to accept cash unless there is a state law that says otherwise.
This is why, when people are pissed about a bill, then try to pay several hundred dollars in pennies, the place can turn the payment down.