Response from paypal is:
Security errorSecurity header is not valid
this means they are not accepting the API Credentials for the sand Box.
double check that you have the API credentials entered correctly, if you are sure they are correct, you may need to request new sandbox API Credentials and make sure to accept the billing agreement.
If you continue to get the Security errorSecurity header is not valid response, contact paypal to have them determine why the API credentials are not being accepted.