First, please double-check if you've entered the bonus code correctly – try to copy and paste it directly into the browser. Also, most codes can be redeemed only once and only for a given time before they expire. If the code has been redeemed earlier or it has expired, you will receive an appropriate error message. Last but not least, keep in mind that once the code has been used, you cannot “reactivate” it, even if you cancel the order in question.
Still, if you have no idea why you cannot redeem your code, contact our Support team via the "My orders and payments" contact form. Please include as much information as possible, including the code that you're trying to redeem, where you got the code from and the contents of the error message.