Security header is not valid in the debug shows that the issue is with the username, password, or access key.
I suggest not using sandbox at all. Sandbox requires a different username, password, and access key and requires you to log in and check out with a sandbox account. Since paypal refunds all fees with refunded transactions, I'd suggest adding a $1 item to the cart so you can test live and just refund the transaction once you get it working.