I have looked at several videos and articles and they all show that there is a key bind assigned to each skill that requires a keybind. As you can see in my attached screenshot, it says in the upper left corner that overhead strike is unbound. There is no "action" button in my settings where you would typically assign keybinds. There has to be some way to assign keybinds in the skill tree.
Ok, the action key name is "Force Attack" and I think default is F key for keyboard and Y button for controller. If you have remapped your keys then this may have been changed or cleared. ALL force attacks use this key in combination with situations and other key presses to initiate the skill. Below are a few pictures of skills and the button combinations required to activate them.
These skills are not bound to a individual key like Slow, Pull, Push, which do indeed have a single bound key to use. These skills are lumped in a class called Force attacks, and are used a bit differently. Your issue is either an out and out bug, OR you have remapped the Force Attack key to something else or have by accident cleared the key binding. So check in your settings, controls and key bindings to which on PC is just below the picture of the controller, to see what has happened if anything to the key binding.
