error in the plugin - paypal adaptive payments

i have the plugin “paypal adaptive payments” in my wordpress , and in this plugin have all my personal details , like my API username , and that the customer buy product in the website of this wordpress , have to customer in the checkout page after that he click on “proceed to paypal” button this error - “An error has occurred while processing your payment, please try again. Or contact us for assistance.” what the solution of this error?
the website -