There are a couple of reasons this can happen, depending on the preferred payment method you have selected in PayPal.
If you are trying to use your available funds in PayPal, please try your purchase in a different browser to see if this resolves the issue. If you are on a mobile device, in an app, such as Facebook, please exit the app and use your mobile browser to complete your purchase.
If your preferred method in PayPal is a debit or Credit Card, please contact your card issuer/bank to confirm your card is enabled to make international purchases. FontBundles and DesignBundles are based in the UK, so oftentimes banks block our transactions, even through PayPal.