ordinal vs twenty-first

ordinal is a noun but twenty-first is not a noun.

Word	Noun	Adjective	Verb	Adverb
ordinal	Yes	Yes	No	No
twenty-first	No	Yes	No	No

ordinal (noun)	twenty-first (noun)
the number designating place in an ordered sequence

ordinal (adjective)	twenty-first (adjective)
being or denoting a numerical order in a series	coming next after the twentieth in position
of or relating to a taxonomic order