Can you make test transaction using the credit card checkout? When you are testing the Express Checkout and you get to PayPal to sign in are you successful in signing in? What happens immediately after you sign in, do you see an order to confirm the details of? Do you get sent back to your site? Also what is the full and exact error you are receiving and what is the URL of the page you are getting the error on? Please also let us know if you are using sandbox or live transactions.