That happens if you aren't connected the the internet, are playing in offline mode, haven't bought the game, or you are logged into another client or game window somewhere else (e.g. another window or a Classic and Minecraft at the same time). If these are all ok exit and reopen Minecraft. It is also possible your account may have been hacked so someone else is logged in as you, if so I suggest that you change your password and try again another time.