unfitness vs inability

unfitness and inability both are nouns.

Word	Noun	Adjective	Verb	Adverb
unfitness	Yes	No	No	No
inability	Yes	No	No	No

As nouns, unfitness and inability are synonyms defined as:

unfitness (noun)	inability (noun)
the quality of not being suitable	lacking the power to perform
lacking the power to perform	lack of ability (especially mental ability) to do something
poor physical condition; being out of shape or out of condition (as from a life of ease and luxury)