substitutable vs substitutability

substitutable is not a noun while substitutability is a noun.

Word	Noun	Adjective	Verb	Adverb
substitutable	No	Yes	No	No
substitutability	Yes	No	No	No

substitutable (noun)	substitutability (noun)
	exchangeability by virtue of being replaceable

substitutable (adjective)	substitutability (adjective)
capable of being exchanged for another or for something else that is equivalent
(of words) interchangeable in a given context without changing the import of the expression