1. 을/를 화장실 is the subject in the sentence, not the object. So object particle 을/를 cannot be used, but subject particle 이/가 should be used. But here, the subject particle 이 can be dropped, so it becomes 화장실(이) 어디에요 or 화장실(이) 어디에 있어요. 2. 어디에요 vs 어디에 있어요 1) For places, both 어디에요 and 어디에 있어요 can be used, and the meaning and nuance are just similar. 2) For people or objects, 어디에요 doesn't work. In that case, 어디에 있어요 is only working. Ex1) Where's your bag? - 가방 어디에요: Wrong - 가방(이) 어디에 있어요: Right Ex2) Where's Mina? - 미나 어디에요? :Wrong - 미나(가) 어디에 있어요? Right Ex3) Where's a rest room? - 화장실(이) 어디에요: Right - 화장실(이) 어디에 있어요: Right

Hi, Camila! 을/를 is the direct object particle. The direct object is the receiver of the action that the subject does. In this case, it's not action It's conjunction. So 을 doesn't fit here. And I would say 화장실 어디에요? or 화장실 어디에있어요? would be the ones that sounds most natural and actually be spoken in Korea. Hope you find this helpful. Good day!