unable vs ability

unable is not a noun while ability is a noun.

Word	Noun	Adjective	Verb	Adverb
unable	No	Yes	No	No
ability	Yes	No	No	No

unable (noun)	ability (noun)
	the quality of being able to perform; a quality that permits or facilitates achievement or accomplishment
	possession of the qualities (especially mental qualities) required to do something or get something done

unable (adjective)	ability (adjective)
(usually followed by `to') not having the necessary means or skill or know-how
lacking in power or forcefulness
(usually followed by `to') lacking necessary physical or mental ability