There hasn’t been an AIR 6.8. If you mean 3.8, you should use a later version, because Apple would reject the app. Version 4 or later would be ok. The current version is AIR 14.
You say that nothing happens, when you touch the icon on your iPad does it become darker, and stay that way? Or does the screen clear, but the app doesn’t run?
If it’s the second option, then most likely there is a script error happening that stops the app before it manages to get going.
If you do a Debug/Debug Movie/On Device via USB, and select your connected iPad, you can then get to see what error is happening.