convertibility vs unconvertible

convertibility is a noun but unconvertible is not a noun.

Word	Noun	Adjective	Verb	Adverb
convertibility	Yes	No	No	No
unconvertible	No	Yes	No	No

convertibility (noun)	unconvertible (noun)
the quality of being exchangeable (especially the ability to convert a currency into gold or other currencies without restriction)

convertibility (adjective)	unconvertible (adjective)
	used especially of currencies; incapable of being exchanged for or replaced by another currency of equal value