"When are we seeing each other?" is a somewhat awkward and somewhat ambiguous sentence in English. It might mean "When are we scheduled to meet?" or when are we scheduled to meet again?" or "When do we wish to meet again?" "When shall we see each other again?" is a more understandable English sentence, but is graded as "incorrect" by Duolingo.
The problem with that sentence is that in English, the verb 'to see' when used to mean 'meet up with/visit a person', is transitive, i.e it requires an object. So it has to be "when are we seeing [OBJECT]", which in this case is 'each other', but could also be 'the movie', 'the letting agent', 'the counsellor', 'the house', 'it', and so on.
I'm not a native Norwegian speaker, but I think the same problem exists - you can't just say "når ser vi" (when are we seeing), because it needs the object (e.g. "når ser vi filmen").
(There is an intransitive use of 'see' in English, but it has different meanings, to do with actual literal sight ("what can you see"), or (more metaphorically) understanding ("I see").)