The game version you are using does not match Blizzard's game servers. Playing in another region that is not your home region may cause this error to occur.
Try switching your region for the game you are attempting to play. This option is found above the Play button.
If this is successful, it means your game does not match the server you were attempting to log in to. Some regions patch on a different timetable, so you must wait until all regions are patched.