Translation:Where are you in the afternoon?
It is were because if you were asking where someone will be in the afternoon it will be 你下午会在哪 the 会 in this sentence means future tense asking where u will be in the future but if you are asking 你下午在哪 it probably means it's already passed afternoon when u are asking this
I think that the question is formed incorrect: If we ask sombody about his or her current location, we sae "We are you?" or "Where are you now?". If the question is about the somebody's whereabouts in the future (in the afternoon that will happen), it should sound "Where you will be in the afternoon?". I am frustrated with the DL English...