One of the most frequent errors on PayPal, this error code is thrown when the transaction is declined at the issuing bank rather than at PayPal. 

Processor declines are a direct result of the financial information entered (because it did not pass an AVS check, if the card has been flagged by the bank and put on hold or if it is not a valid card) and are not under PayPal's control, so the only way to understand the cause of the error is to contact the issuing bank. 

In the interim, you can try asking your customer to use a different card to complete the transaction in question.