ability vs unable

ability is a noun but unable is not a noun.

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

ability (noun)	unable (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

ability (adjective)	unable (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