If you cannot start a game it may be that your licence has expired. A game can be started only with a valid licence. You can check your licence validity in your profile settings. In case your licence has expired and you need to start a game you can add an additional Game Instructor (a valid licence holder) who can then start the game.
If your licence is valid and the game still won’t start, this may be due to a temporary technical error. In this case try reloading the browser and/or logging out and back in again. Another reason may be a lagging internet connection.