Modern Warfare is rated at 17+. If your profile was setup to only play games rated below that rating, it will stop you from being able to play that. There are parental controls associated with profiles that can be setup to only allow certain profiles to be able to play certain rated games.

Clear the Cache of Your Console. Another common reason for the Call of Duty Modern Warfare Multiplayer not working error is the corrupted cache data. In this case, you can try clearing the cache files on your PC or console to fix the issue.