Issue
PayPal returned API error code 15004 - Gateway Decline - This transaction cannot be processed. Please enter a valid Credit Card Verification Number.

Cause
The Credit Card Verification Number (or CVV, Card Verification Value) entered is different from the one on the credit card. This error also occurs when the card's expiration date is incorrect.

Solution
Here's how to resolve this issue:

• Check that the credit card's CVV number was entered correctly. The length of the CVV number depends on the credit card type:
• Visa, MasterCard, and Discover - three digits
• American Express - four digits
• Make sure the credit card's expiration date was entered correctly.
• The expiration month must contain two digits (MM) between 1 and 12. Include a leading zero, if necessary (for example, 05 for May).
• The expiration year must contain four digits (YYYY) and be equal to or greater than the current year.

See also:
Payflow Gateway Developer Guide and Reference

See the Developer Portal for a complete list of NVP/SOAP API error codes.