That's coming from PayPal (https://developer.paypal.com/docs/cl...pi/errorcodes/), most likely coming from the setExpressCheckout API request.

You could try enabling the PayPal payment-method's debug -- although if you're really still using Zen Cart v1.3.9a, I'm not sure how much information that log's going to tell you. The Zen Cart PayPal Express Checkout payment method has gone though some significant changes in the last 5 1/2 years.

The logs, when generated, will be in the /includes/modules/payment/paypal/logs folder.