Classifieds Checkout Array Warning

This is the warning I get when on the 'Classifieds Checkout' page after choosing PayPal as my 'payment method.'

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

I am in Sandbox mode.
Where is the error I may be making?
Thank you.