"You are impossible!"
Translation:Du är omöjlig!
The "T" at the end of an adjective, in this case "Omöjligt", means it can only be used with "Ett" nouns.
"Det är omöjligt" - "It's impossible".
When you're describing a person or an "En" word, then it becomes "Omöjlig".
It has nothing to do with the position it is in a sentence.
du is used for singular and ni is used for plural. However, some people use ni as a ”polite pronoun”, so Ni är omöjlig should have been accepted (I've added it now).
The 'polite' usage of ni is a bit controversial: most people in Sweden don't do it, and some may be offended by it. But it seems to be quite normal in Finland. See this discussion, including the comments: https://www.duolingo.com/comment/5591933