PayPal doesn't work

-- url of the page with the problem -- : la-dune-argentee.fr
-- HikaShop version -- : 2.3.4
-- Joomla version -- : 3
-- Error-message(debug-mod must be tuned on) -- : The operatoon can't succeed.

Hi

I have a problem with PayPal. When customers pay with PayPal after validating their informations, PayPal refreshes and says that the transaction can't succeed.
Need help please.
Regards