Fernando is right. The put is a contract that allows you to sell shares to the counterparty at a set price. However, you have to own or borrow actual shares in order to sell them (you can't sell phantom shares to someone that you just conjured up from out of nowhere--if you did, what would they really own?).
So, if you don't own shares, and the broker can't find a borrow for you (or isn't willing to violate REGSHO), then you're up shit's creek basically. The lower quality brokers seem to be better here, as they'll just allow a naked short.