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”

lacking the necessary skill or knowledge etc.

• “an incapable helper”

(followed by `of'') not having the temperament or inclination for

• “simply incapable of lying”

not meeting requirements

• “unequal to the demands put upon him”