Why can't I use thousand cuts? I have all the skills for it.

@deathssavior1577 As others have said, it is a focus based skill (like your rift ability), you must have at least one full focus bar to use it. Whether you're using a bow or daggers does not matter. Focus abilities are not intended to be able to be used often, you save it for when you really need it. There are also inquisition perks (from the war table) to increase your focus bars from 1 to 3.