This question comes up often. I think it is a quirk of English and not Hungarian.
Casual English adds a definite article to elevator (referring to "the elevator" or "the elevators") as if all elevators in a single building/location are the same object. "An elevator" means one of many elevators not specific to the current location.
So one would say: "I am waiting for the elevator" when standing in front of three elevators, and say "I am in an elevator" if talking on a mobile phone to someone not in the building. But one might also say "I am in the elevator in this building" when talking on the phone to someone in the same building (to be specific about the group of elevators), or say "I like to ride the elevator in this building" even if there are many elevators.
The definite article is also used for other kinds of travel which are all identical ("I am going to go on the boat", "I am going to go on the train", "I am going to take the bus") when there are many identical boats, trains, or buses and you will travel on one of them, and it doesn't matter which specific one.
Also, this is in casual english only. Formally, definite/indefinite articles apply to "elevator" the standard way.
This question comes up a lot. Officially, the course is in American English (I believe all English Duolingo courses use American, both to teach English other languages)
Now, I am not opposed to adding British spelling and vocabulary as alternative translations. And I think they should, within reason. You can report your answers as "should be accepted"
But I also see why they don't prioritize this. When the course is in beta and still has lots of bugs. It's a lot of work to just add all the American acceptable translations. And why stop at British, we might add Canadian, Australian, South African, Indian, etc variants too.