accessibility vs unaccessible

accessibility is a noun but unaccessible is not a noun.

Word	Noun	Adjective	Verb	Adverb
accessibility	Yes	No	No	No
unaccessible	No	Yes	No	No

accessibility (noun)	unaccessible (noun)
the quality of being at hand when needed
the attribute of being easy to meet or deal with

accessibility (adjective)	unaccessible (adjective)
	capable of being reached only with great difficulty or not at all