why is "this is not a juice" not accepted? I don't know whether it should be or not
It's because 'це' translates to 'this is' (in this sentence, anyway), not 'that is'.
It is not juice is also correct. It seems that duolingo sometimes does not accept contractions in English, although it has accepted Where's in an earlier lesson. Probably best not to use them to avoid being marked incorrect.