Due to recent changes by Ubisoft and third party services, the platform Uplay(where you used to play Ubisoft titles years ago) has been shut down or at least its servers, therefore all multiplayer and online services for games that used this platform are completely unusable(At least in an official way). The purpose of this guide is simple, make the game playable.

1. Go to your Steam Library and locate Assassin's Creed Brotherhood.
2. Right click on it and select properties.
3. In the General tab go down to Launch Options and type -playoffline
And that's pretty much it, enjoy your game :D

This workaround gives you the ability to play the game on single-player, as for multiplayer or online functions like extra content and DLC unfortunately this method won't help you with that, but since this is and old game and you most likely just want to play the campaign, it's not really a problem.