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