When you pay with your credit card in a shop you confirm your identity by signing the receipt and showing your ID. Since this is not possible for online payments, credit card issuers have introduced “3-D Secure” as an additional security layer (called SecureCode by MasterCard and "Verified by Visa" by Visa). This is an additional safety measure, which requires you to enter a secret that is only known to you in order to prevent unauthorised payments.
Unfortunately, what to check and how to check is up to each card issuing bank: Some banks send a text message with a transaction number or offer special mobile applications, other banks provide TAN generators or cards or use a password that you chose when making your first credit card payment on the Internet. For that reason, we cannot help you when you encounter problems with your credit card payment. We suggest to get in touch with your bank in order to understand how to complete your online payment.