Translation:The Nordic countries
Eh... I mean, I've seen Duo take it in other places, but from my understanding "Norden" has a more specific meaning than "the North" does in English. "Norden" is really a geo-cultural region containing Finland, Sweden, Norway, Denmark, and Iceland, as well as their autonomous regions, Greenland, the Faroe Islands, and the Åland Islands. In contrast, "the North" (to me at least) would suggest a larger area that includes Canada and Russia, as well; and it emphasizes the geographic side of things, while Norden is definitely united by culture and not just geography.
Hope that helps.
To some extent it would depend on who you ask, but I think generally speaking, Scandinavia is taken to mean the three countries of Denmark, Sweden, and Norway. It excludes Iceland, which is why you might think it means the mainland, but it also excludes Finland and includes those islands which are claimed by Denmark, Sweden, or Norway (which is about half of Denmark's territory).
Like I said, I've seen Duo accept it. In fact, it's probably accepted in most or all of the sentences that use Norden. The answers that always gave me grief were the Nordic countries/region. That's the more accurate answer, but sometimes Duolingo would accept one, and sometimes the other. I think that's been fixed now.
In this case, "the North" has a geo-cultural component, much like a resident of Virginia in the U.S.A. would refer to northern parts of the country as "the North." An example in Sweden would be "Lejonet från Norden" (Lion from the North) which is echoed in the southern Germanic tongue "Der Löwe aus Mitternacht" [Lion of Midnight (lands)].
'tis indeed! Typically, "Scandinavia" refers to the three countries of Norway, Sweden, and Denmark. "Norden," on the other hand, includes Scandinavia and also Finland, Iceland, Greenland, the Åland Islands, and the Faroe Islands. Both are sort of geographic/political/cultural terms, but one is much narrower than the other.
Scandinavia is actually not very used in the Nordic region. There is no consensus as to what it entails. Is it Sweden and Norway; Sweden, Norway & Denmark; or Sweden, Norway, Denmark & Finland?
Depending on who you ask you will get a different answer. Therefore, "Scandinavia" is quite a problematic concept. "Norden", however, is not.
It's probably best to think of Norden as a proper name. When you hear Norden, it will (or should, at least) always be a reference to a very specific grouping of countries: Sweden, Denmark, Norway, Iceland, and Finland - plus all of their colonies, that are at various levels of autonomy. Norden will always be definite (like the United States is always definite), and will never be used by a native speaker to mean anything other than that specific geo-political region.