Paypal express set up

Hi. I am trying to set up paypal express on this site. I obtained paypal credentials from a paypal sandbox account. I entered all this into the marketpress payments set up switched to live and now I get the following error-
There was a problem connecting to PayPal to setup your purchase. Please try again.
10002 - Security error - Security header is not valid.
I have looked through the community forum but can't see the exact problem that I am having there so would appreciate some help. Thanks