Nubian vs ethnic group

Nubian and ethnic group both are nouns.

Word	Noun	Adjective	Verb	Adverb
Nubian	Yes	No	No	No
ethnic group	Yes	No	No	No

As nouns, ethnic group is a hypernym of Nubian; that is, ethnic group is a word with a broader meaning than Nubian:

Nubian: a native or inhabitant of Nubia
ethnic group: people of the same race or nationality who share a distinctive culture

Other hypernyms of Nubian include ethnos, Egyptian.

Nubian (noun)	ethnic group (noun)
a native or inhabitant of Nubia	people of the same race or nationality who share a distinctive culture