This is usually a temporary thing! Clickbank deals with thousands of transactions every day so has a massive server load and is therefore prone to getting overloaded. I would recommend either trying again within a few minutes and if that doesn't work, using one of the other payment options listed below and emailing us about it.
Clickbank has some automated security features when it comes to credit card transactions, especially with overseas credit cards. For instance, if the country of origin of the credit card and the country where the Kingdom of Pets product is being ordered from are different then it is likely that the transaction will be denied. I would recommend using one of the other payment options listed below and emailing us about it.