Many players struggle with withdrawing the bonus they claim. This is usually due to one of these reasons:
The wagering requirement that hasn't been fulfilled
The minimum withdrawal amount is higher than your balance
The use of different withdrawal method than used for the deposit
Most bonuses come with wagering requirements (and other terms and conditions from time to time). To be able to withdraw your casino bonus, you need to wager the money a number of times. 30-50 times are common.
If you have won €15 and want to withdraw casino bonus into your bank account, it might be hard. Some casinos have €20 as a minimum withdrawal limit (some have €10 and some more than €20). Different payment options do, from time to time, offer different withdrawals limits as well.
At some websites, you need to withdraw your casino bonus (and other winnings for that matter) to the payment options you made a deposit from. If you made a deposit from a VISA credit card, you need to make the withdrawal to that same VISA card again, as an example.