So in the case of MCC's matchmaking, this means an update to how the game connects to other copies of the game and Xbox Live. There may not be enough players to support a game in this playlist.
Mercy is a beautiful new Arena multiplayer playspace that combines contested corridors with a mix of open sightlines. With the new Forge Content Browser for Halo 5: Manage your fireteam in the Roster and try again. We've encountered a problem and couldn't join you to the game. This means that a mandatory update is available for the game, and the latest version is required for you to continue. We'll have even more details to share later this week on HaloWaypoint.