"The pharmacist is working at the pharmacy."
Translation:약사는 약국에서 일하고 있어요.
I'm only learning still too, but I think because the act of possession is not an ongoing thing, you would use 약국'에' instead of '에서'
Or maybe another idea is that because in the English sentence you seem to be emphasizing the fact that the medicine is specifically possessed at the location of the pharmacy - you would use different particles for that reason to emphasize that also in the Korean? Like maybe somehow 약국은 as the topic