The one just released for Prepar3d, QOTSII.
Then be sure you have installed the latest GSX, including the current Live Updates (which are applied either when installing, or when notified in the sim), because the airplane is surely supported.
I tried it just now, and it works normally.
Some other thing you should check, is if you have the Verbosity switch slider in the GSX Settings all the way down. With this setting, which is not default, GSX won't tell you anything so, maybe there's a reason why you cannot start pushback, and GSX would have told you, but you don't see the message because you turned down all the messages with the Verbosity slider.