Learn Hardcore Finnish: Missä on kattilan kansi?

Missä: the question word meaning "where (in/at)"; it's in the inessive case.
on: 3rd person singular of olla "to be" (is).
kattilan: genitive singular of kattila "pot," marking possession ("of the pot").
kansi: nominative singular of kansi "lid"; it's the item being located (the subject/complement of the sentence).

Why is kattilan in the genitive (-n) case?

Finnish uses the genitive to show possession or an "of" relationship. kattilan kansi literally means "the pot's lid" or "lid of the pot," so kattilan must be genitive.

Why is kansi in the nominative and not genitive (kannen)?

Because kansi is the thing whose location is being asked about—it's the grammatical subject/complement of the clause with olla ("to be"), so it appears in the nominative. The genitive kannen would be used if kansi itself were the possessor of something else (e.g., kannen kahva "the lid's handle").

Could I say Missä kattilan kansi on? instead?