Why did I get API error code 10406?

The PayerID value (Unique Account Identifier) is invalid. You may receive this error message when performing the following API calls:

• DoExpressCheckoutPayment.
• DoUATPExpressCheckoutPayment.
• DoReferenceTransaction.
• RefundTransaction.

To resolve it, check that the PayerID value in your API call is correct. The PayerID should contain up to 13 alphanumeric characters.

PayPal NVP and SOAP API Reference Guides.

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