In my family, we've always used the word "сок" for "juice" and not "сік".
This goes back hundreds of years.
Has anyone else out there use "сок" instead of "сік"??
Just curious.
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.