The online ticket office uses a payment system which complies with the "3D Secure" standard, intended to make online payments even safer.
After validation of your bank details (credit card number, expiry date and card security code) on the payment page, you will be redirected to a form from your own bank, asking you to enter a secret code known only to you. This code has been sent to you by your bank.
The payment will only be accepted after validation of this code by your bank. The format of this code varies by bank e.g. date of birth, numerical code, alphanumerical code, etc. Some banks send this code via an SMS message, which is sent to you as you are about to pay.
If you do not know your code, contact your bank to obtain it.