Also you can stand "at the back"
I agree with you Judit - at the back is more common.
When translating elöl to English, do you always have to include "the" as in "the front"?
Probably yes. Sometimes you can also say "up front".
In this context, "in the front" would match "in the back". Duolingo wants it both ways!
An Englishman would always say "at the back", not "in the back". "In the back garden" is OK, because here "back" is used only as an adjective.