I won money using my sign-up bonus, but I can not cash it out now. Why?

Almost all sign-up bonuses have a wagering restriction imposed on them. Most often this will mean that you must wager 20 up to 50 times the sign-up bonus amount in order to be able to cash out any winnings. Others allow cash outs up to a certain amount or just zero out the bonus after a certain time frame. Be sure to carefully check the sign-up bonus conditions of the casino, so that you know what to expect.
On the other hand, such restrictions are somewhat understandable, because the casino does not want thousands of new players just to sign in, deposit, and then cash everything out without placing a single bet.