This error typically occurs when the verification code entered does not match what is on the card.
The exception should be logged in the exception.log file as follows:
exception ‘Magento\Framework\Exception\LocalizedException’ with message ‘CVV2 Mismatch: 15004-This transaction cannot be processed. Please enter a valid Credit Card Verification Number.’ in vendor/magento/module-paypal/Model/Payflowlink.php:388