Can prepositional phrases be subject complements?
Prepositional phrases, because they're a type of phrase where a verb, or adjective, is used to alter an adverb or noun, they count as subject complements. Subject complements, words or phrases that modify, describe, or complete the grammatical subject of a clause, are then technically similar in function to prepositional phrases.
"He's in the kitchen", with that being said, this phrase can technically count as both a prepositional phrase, and a subject complement