Why can’t I use one of my skills?
Skills require a specific amount of Rage. Make sure that you have collected enough Rage to cast it, and that the Cooldown is not active.
You can see your Rage in an orange Rage Meter. Most Skills cost Rage, however, the basic "Auto" attack and the basic Class Skill increase Rage instead of using it.
A character with a high initial Rage Level is most likely to win in a fight, as he or she will be able to use more powerful skills from the beginning.