However, incapable is the proper and original form, and furthermore, everyone uses it. I have never seen uncapable in use. The rule of thumb to go by is: If you're choosing between variants of a word, pick the variant most widely used and understood. So when you have to choose, choose incapable.

Is incapable a noun or verb?

—incapability /ɪnˌkeɪpəˈbɪləti/ noun [uncountable]GrammarSomeone is incapable of doing something: He is incapable of lying.

What is the difference between incapable and incompetent?

As adjectives the difference between incompetent and incapable. is that incompetent is unskilled, lacking normally expected degree of ability while incapable is not capable (of doing something); unable.