"I eat that in the bathroom."
Translation:저는 화장실에서 그것을 먹습니다.
You should really read this. Duolingo Regular Verbs
Similar to ~으로, ~에/에서 can have another meaning when not used with a verb of motion.
Both ~에/에서 in this case mean in or at.
~에서 where an action takes place
~에 where something static is happening
So in this particular case,
저는 화장실에서 그것을 먹습니다.
화장실 – bathroom
에서 – in, at
에서 – from (with a verb of motion)
Eating is an action, not something static and isn't a verb of motion so 에서 must be used. And it means in, not from.
화장실에서 – in the bathroom.