Purchase Credit API Paypal doesn't run

Hi, I have a problem when user try to Purchase Credit ...
At the checkout when I try to connect with PayPal ... it give me error:

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

What I have to do ??
I have Insert my API user and Password