incapable

(followed by `of') lacking capacity or ability; "incapable of carrying a tune"; "he is incapable of understanding the matter"; "incapable of doing the work"

not being susceptible to or admitting of something (usually followed by `of'); "incapable of solution"

(followed by `of') not having the temperament or inclination for; "simply incapable of lying"

not meeting requirements; "unequal to the demands put upon him"