You can’t connect to the Sandbox Environment, because Sandboxes are disabled by Apple.
From [Apple][1]:
As part of the release of iOS 9 and El Capitan, the Game Center development sandbox has been eliminated, allowing all pre-release development to occur in exactly the same server environment as released games. This change brings several advantages including:
- Simplifying update testing by allowing testers to play with the full player base, not just other testers
- Removing any implementation differences between the sandbox and release environments
Can you upload an image of the error that you’re getting when you’re rejected, or the message provided?