"There are a lot of mountains in Austria."
Translation:Det finns mycket berg i Österrike.
We do that sometimes, when the noun is countable but there are too many "items" to count them all really:). Here, the mountains are like a lot or a mass. "Mycket berg" seems slightly coloquial though and "många berg" should definitely be accepted. If it's not, you should report it.
I am confused why “många” has to be checked here… Duolingo is sometimes very strict about translations, but not here… Until now I figured that “a lot of” = “mycket” and “many” = “många”. I understand that the meaning of the sentence does not change, but these are NOT the exact same words.