"juice of an apple"

Nope, it would just sound weird. Inverting the order of a here is, probably, only ever performed in poetry. Placing the adjective before the noun and its attribute in Genitive after it is followed rather strictly in plain speech and writing of a vast range of styles.

On the other hand, I am not realy sure why do we need THIS in the course. It is not that you would see «сок яблока» anywhere except, maybe, in a listing of ingredients on a box of a juice or something that contains it.

If you wanted to put it in that order it would be яблочный сок (apple-ish juice or apple-flavored juice). That would be turning яблоко into an adjective instead of declining it. That was in the reverse course.