PayPal Error 15004

Paypal API
The PayPal API, or application programming interface, is a series of tools that you can integrate with your website. With the PayPal API, you can allow your visitors to purchase items from your site with their PayPal accounts without ever having to leave your site. Because potential customers do not have to go to a third-party site to pay, they may feel more comfortable about divulging their personal information. In addition, with the API, you can design your own checkout page, and you are not stuck with PayPal's design.
API Security
One of the benefits of the API is its integrated security features. In addition to the fact that the API transmits data using Secure Socket Layer technology, it allows you to earmark suspicious transactions for further scrutiny and provides you with an API certificate that you can use to connect to PayPal securely. It also provides you with a comprehensive library of error codes and their meanings. You can use this library to diagnose problems when you see that someone was unable to purchase from you using their PayPal account or their credit card.
Error Codes
These automated error codes protect you from a wide range of potential problems. Aside from the obvious issue of someone not entering a valid debit or credit card number, you may run into several other potential issues. For example, you will see error 10407 in your logs if a potential customer enters an improperly formatted email address into an order form. Some people do not like giving their information away, and they will enter fake email addresses instead. You will see error code 11455 in your logs if your prospective customer got as far as the billing agreement and then did not click the "I agree" box. In this case, the transaction would have ended there.
Error 15004
You will see error 15004 in your logs when someone tried to sign up for your service or buy your product with a debit or credit card, but were unable to correctly enter the three-digit CSV code located on the back of the card. The CSV code is an extremely important security feature because it increases the chances that the person making the order possesses the physical card being used. PayPal calls this type of error a "Gateway Decline," because the person attempting to use the card will be able to go no further in the purchase process.