twenty-six vs xxvi

twenty-six is a noun but xxvi is not a noun.

Word	Noun	Adjective	Verb	Adverb
twenty-six	Yes	Yes	No	No
xxvi	No	Yes	No	No

As adjectives, twenty-six and xxvi are synonyms defined as:

Other synonyms of twenty-six include 26.

twenty-six (noun)	xxvi (noun)
the cardinal number that is the sum of twenty-five and one

twenty-six (adjective)	xxvi (adjective)
being six more than twenty	being six more than twenty