Both are used similarly in some context and can be replaced, but there are differences between them. For example, ~에 Where is my bag? 내 가방이 어디에 있지? >> sth placed or place ~를 Where are you looking at? 어디를 보고 있습니까? >> direction So 직장에 다니다, 직장을 다니다 both expressions are possible to use, but technically a structure of the sentence are slightly different in terms of grammar.

다니다 means comes and goes (attend) to somewhere regularly or during a certain period. So it's used when you come to somethere with purpose, for example learning, practicing, working, or whatever specific reasons.

So these are very common examples you can hear in Korean.

학교에 다니다. School, any kinds of educational organisations can be subject like daycare, kindergarden, elementary/middle/high school, college, university

학원에 다니다. Institution

직장에 다니다. Workplace

헬스클럽에 다니다. 수영장에 다니다. Gym, or any kind of sports activities and class you're attending