From time to time your bank will decline your card when they do not recognize a merchant, or place a hold on the card for a security check and to verify the transaction. When this happens your card can be declined even if you have enough money in the account and have triple checked the details.
When this happens you may need to get in contact with your bank to ensure they know it is you making the transaction. An alternative is often to select PayPal instead. With it’s worldwide recognized brand, banks often see this as a legitimate transaction and allow you to process the order.
To select PayPal instead you can do so as per the image below