This may be because you have not yet played enough to have met wagering requirements. If you accept a deposit bonus or receive any free monies/bonuses from them, you must meet wagering requirements before you can withdraw. This is easily achieved by continuing to wager on slots.
To check whether you have met wagering requirements, simply click ‘Cashier’ under the menu, select ‘Withdrawal’ which will reveal the amount you are required to wager before completing a withdrawal.