You can only complete the purchase process online if all the data for the means of payment have been entered correctly. Check your details for spelling mistakes and make sure that the details of the CVC code (3-digit code on the back of the card) are available when paying by credit card. If payment by credit card is refused, it may be due to the credit balance, pay with another card if necessary. Sometimes it is also due to the browser, use a different one if necessary.