Why did I get API error code 10401?

The OrderTotal (SOAP) or AMT (NVP) variable in a DoReferenceTransaction, SetExpressCheckout or DoDirectPayment request is missing or invalid.

To resolve this error, check that the OrderTotal or AMT variable is present and greater than zero in your request.

PayPal NVP and SOAP API Reference Guides.

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