With the release of macOS 10.12, Apple has disabled Game Center for all 10.8.5 and newer Macs. With that, multiplayer for C&C is disabled and is a single player game only and has been discounted at 50% off.
We've submitted the issue with Apple and are awaiting an update from them to resolve it.
In the meantime, please log out of your Game Center app account and disconnect your internet connection.
Relaunch the game and it should then load properly.
Please "follow" this article for updates as they are available.