Get an error when trying out a payment to PayPal sandbox using coursepress/marketpress

Hi
Trying to set up PayPal with the developer sandbox. I've entered the API credentials but on testing get a message that says "Oops! An error occurred while processing your payment." in a red box. There is no other clue as to what the issue is. Am I doing something wrong. I've checked the API stuff several times but always the same result.

Thanks

john