There might be several reasons for a payment to not go through:

1. You don't have enough funds in your account.
2. The card exceeded the online purchase limit.
3. Your card doesn't have 3D Secure enabled or 3D Secure failed.

While the first two points can be easily checked and overcome by using another card, the 3DS verification can be quite finicky.

3D Secure - it's an authentication step for online payments and it provides an additional security layer for online purchases made with credit and debit cards. It's known as VisaSecure for Visa, SecureCode for Mastercard:

If you enter the payment details and receive a "Payment could not be processed" error, try the following:

• Check if you have a pop-up blocker installed on your PC. 3DS opens up in an additional window that is easily blocked by pop-up blockers. Disable the blocker for a few minutes and try to pay again.
• Check with your bank that your card is enrolled for 3DS.
• Try another payment method such as PayPal (with or without a PayPal account) or a wire transfer.

To pay without a Paypal account select "PayPal CheckOut" then "Pay with Credit Card" on the PayPal payment page: