If you are getting a security header invalid message it means that PayPal is not allowing you access to the api based on the credentials you provided. The problem could be a billing agreement that needs to be confirmed in your PayPal profile. It could also be that your credentials are not correct. You should check your profile to ensure that there is not something preventing you from using the credentials. If you continue to have this problem you should try requesting new credentials then make sure to cut and paste them into the server behaviors.