API error code 10443 can be caused by:

• Setting the parameter PaymentAction to "Sale" with SetExpressCheckout at the first API call.
• Not granting a third-party the necessary permission to make API calls on your behalf.

If you get this error, try the following:

• Use the same PaymentAction for the DoExpressCheckout API call you used in the initial SetExpressCheckout API call. PayPal doesn't allow the switch from "Sale" to "Authorization" in a single checkout session.
  Note: If you receive this error at the DoExpressCheckout API call, set PaymentAction to "Authorization."
• Grant the third-party permission to complete the API call. If you’ve already granted the third-party permission, ensure you used their correct username and selected the required permissions.

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

See also:

Add New Third Party Permissions