Romanian is not out yet :)

'cause juice is a mass noun. So, "this is not juice" is accepted

It does a bit.

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.