Questions about the gameplay of Heroes 6.

I developed the "Heroic Onslaught" skill for my hero. However, I couldn't use it even once during battles due to a technical issue. It was selected in the spellbook, but when I tried to apply it to my unit, a hand with a red circle appeared, and the skill couldn't be applied to any unit. Why is this happening? The hero was developed along the Path of Tears. Am I correct in understanding that he shouldn't follow the Path of Blood in order to use this skill?