PayPal security error in the signup process


I am trying to set up classifieds and directory plugin . I put in the paypal API signature into the settings, but when I try to test out the signup from the front end, I get the following error.

Signup ( error )
Security header is not valid
SetExpressCheckout API call failed.
Detailed Error Message: Security header is not valid
Short Error Message: Security error
Error Code: 10002
Error Severity Code: Error