This error occurs when a transaction is declined at the issuing bank. While it can materialise for a number of reasons, the general causes are issues like insufficient funds, a frozen account, an invalid credit card number or an expired card. There is no way to pinpoint the exact cause of the error without contacting the issuing bank but it is important to note that none are the result of an oversight on your part.

That being said, it is possible that the error has popped up because you have hit an amount transaction limit (either a monthly amount limit or an individual transaction amount limit) set in place by your merchant service provider You can find out more about card declined by issuer errors here.