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In Cyradil is the Ability bar bugged or what? -NON VET
You have to be close enough (8 meters) to an enemy target for it to be available. Could that be the problem? If not, I would double check that you have enough magicka to cast it (though I doubt magicka is the problem as it isn't that high of a cost). If that it's not a magicka or range problem, I would say it bugged out. I have never seen or heard of that bug before, but you could try logging out and back in to see if that fixes it. If you do it within a few minutes you should still be in Cyrodiil and not have to rejoin a queue to get back in to try it.