Because of security reasons, PayPal does not always accept international (non-US) credit cards. We use PayPal for all payments, even if you do a credit card checkout through the site, it is still going through PayPal which is safe and secure. PayPal has various reasons for not accepting all cards, and generally if it doesnt go through it gives the same error. If there is not enough money in the account, the card has been stolen, or if it is just an international card, it gives the same non-descript error. So if you get one, and are using a non-US credit card, this is probably the reason. It may work if you go through the full PayPal checkout process instead of using PayPal Express, it may not. If it doesnt, then the only way is to use a different card, or to transfer money to the PayPal account and then make payment.