Actually you can totally say I want a juice, or Do you want a juice? Because you are talking about a container or a glass of juice, but you can't say I want a sweet juice. Because sweet juice is not a name of any kind of juice, kinda. It gets more uh complex, not really complex, but different, in english. Because you can say I want a sweet juice, or the girl wants a sweet juice. If you name all sweet juices sweet juice. And say she wants one of those juices. Really what i am saying is not part of the real rules of english just dilectical speech. But you can totally say "she wants a sweet juice." It probably should be correct.