Heh, I also struggle with translating -n as an accusative marker, wanting to translate it instead as a definite marker as in DA/NO/SV, and expecting -m to be an accusative marker as in Latin (I gather Esperanto takes its -n marker from Greek).
Add to that translating "ni" as you (it's so in Mandarin, also), and Esperanto is proving more of a challenge than I expected - not because of any actual difficulty, but just from replying more quickly than apparently I should :p
I only did the placement test for fun to see how far I would get. :)
My opinion on Nynorsk is that it is a good attempt at bringing our language closer to the dialects and also how Norwegian was before our union with Denmark, but as it is today it is too far from how Norwegian is spoken in all other parts except for the western parts.
I don't think I've ever heard a native English speaker say that, so I'd say yes that's wrong. But if you're a native speaker that would say that then you may have to chalk it up to Duolingo really only accepting Standard English. Lots of my dialectal English translations aren't accepted.
I am a native, and sign as a noun can mean two things: Something telling people what to do/what is there (e.g. a sign out the front of a shop advertising a special, or a road sign which tells you to do something such as keep left/right). Secondly, a sign as in a component of sign language. Signature is the noun you want in this context.