affirmative vs approbative

affirmative is a noun but approbative is not a noun.

Word	Noun	Adjective	Verb	Adverb
affirmative	Yes	Yes	No	No
approbative	No	Yes	No	No

As adjectives, affirmative and approbative are synonyms defined as:

affirmative and approbative: expressing or manifesting praise or approval

Other synonyms of affirmative include approbatory, approving, plausive.

affirmative (noun)	approbative (noun)
a reply of affirmation

affirmative (adjective)	approbative (adjective)
affirming or giving assent	expressing or manifesting praise or approval
expressing or manifesting praise or approval
expecting the best