You may not be able to withdraw your balance when using a bonus, due to wagering requirements.
These requirements specify how many times you need to wager the bonus amount, before it becomes withdrawable. You can track your wagering progress under My Account → Promotions → Active Promotions.
Bonus funds are kept separate, and to convert them to real money, you must meet the wagering requirements. For example, a $100 bonus with a 10x requirement means you must bet $1,000 before withdrawing.
When you receive a bonus, the funds are added to a separate bonus account balance.
*The data used in the screenshot are indicative.
Be sure to review the Bonus Terms to understand all conditions.