Hi!
Do you receive this error message when you go live with Paypal? And I expect that if you use it in test mode, Paypal Express Checkout works properly.
If so, please try to apply the following change. Replace the
Code:
protected $apiLiveURL = 'https://api-3t.paypal.com/2.0';
line with the following one:
Code:
protected $apiLiveURL = 'https://api-3t.paypal.com/nvp';
in the classes/XLite/Module/CDev/Paypal/Model/Payment/Processor/ExpressCheckoutMerchantAPI.php file.
Then, you should re-deploy the store and test your Express Checkout again.
Please, let me know if it works out.
Tony.