Translation:Priests work in churches, while teachers work in schools.
The sentence has a general meaning. "The church" & "the school" should be accepted anyway.
A delfin nagyon okos állat. = The dolphin is a very smart animal. OR Dolphins are very smart animals. (OR A dolphin is a very smart animal., although, I don't think this course should accept this translation.)
The problem with this is that it only has a general meaning, if there is no "a". By saying "a templomban", we are saying that "in the church", meaning a specific church, instead of generally "churches".
The priests work in the church and the teachers work in the school - not accepted - reported
a papok = the priests a templomban = in THE church a tanárok = the teachers az iskolában = in THE school
If Duolingo would like to explain somewhere in this course why/where/when you can exclude "articles" then maybe us students would know to do that !!