During MyPrize's beta period, the maximum bet amount is 2 USD per round when Betting Together or 20 USD per round when betting solo. If you refresh the page and reduce your bet amount to less than the maximum, the error message should resolve.
For bonus buys when you are betting solo, the maximum bet amount is 200 USD. Since the bet amount is 100x the amount you enter, the maximum you can enter is 2 USD.