Some players who have farmed the 10x Skyshards required to get the mount from Alani has reported problems with actually tagging her, and instead of being able to attack her when using the Skyshards, they get the error message "You can't do that yet". This is obviously very annoying for the players who have just spent a long time trying to get all the Skyshards, but there is kind of a workaround to this bug.
In order to be able to tag Alani, you will need to find her on another server where she is not already tagged. If you can't tag her, then first try to turn Warmode on/off, and if that does not work, use an add-on to change to another server. Keep hopping servers until you find one where you are able to use the Skyshards and tag her.
So what causes the bug? Apparently it has to do with players being able to tag the mob without killing it. This will put Alani in a weird position where no one else can tag her until she is reset. So there is not much to do about it really, except either do the method above, or ask a GM to reset her for you.