Fjäll is specifically used to refer to the mountains between Sweden and Norway. They could also just refer to mountains that reach above the tree limit.
In the sentence of the topic they most certainly mean the mountains between Sweden and Norway and the speaker is probably going on vacation either hiking (in the summer) or skiing (in the winter).
Ett fjäll is a flat-topped mountain that reaches above the tree line in Lapland (which streches across Norway, Sweden, Finland and Russia) or in Scotland. 'A fell' is a good alternative translation if one wants to make a distinction between ett fjäll and ett berg. The Finnish Father Christmas (Jultomten in Sweden or Julgubben in Finland), by the way, does not live on the North Pole, but in the Finnish Lapland on a fell called Korvatunturi (tunturi = ett fjäll in Finnish).
I do see your point, but there are many situations here where the point is to teach the word in isolation rather than contextualised. This is largely because Duolingo's backend system doesn't allow for distinguishing between accepted solutions in such a way that the feedback given when making errors is always usable. I'd accept "travel" if it worked slightly differently. As it is, if you enter e.g. "We go the to mountains" by mistake, the system might tell you that no, gå means "travel", not "go". This happens all of the time if you're not careful as a course contributor. :(