元気な　 人 　　← correct. (adjective)(noun) 元気(genki)　人(hito) 　← wrong. (noun) (noun) 元気人(genki jin/bito) 　← as a slang or a unique noun, it's okay. Actually, according to some dictionaries or a grammar site, 元気 is a noun/an adjective verb(形容動詞)/an adjective, but 元気 is a noun, 元気だ is a 形容動詞, "元気 itself is an adjective" is wrong information.

「元気な人」です。

「元気な人」です。