Hard to say. Could be either as I wouldn't expect grammar to be fully correct in a fast food establishment, especially if it's temporary.
If cards are not accepted at the register, cash only is appropriate. However, of the.kiosks don't allow cash, then cash can only be used there.
If it's the former, it would be easier to say no card payment at register, or change the word order to 'Only cash taken at register' If it's the latter a comma would help: 'Cash, only at register'.
Cash,