This statement, "Make juice," has a subject and a predicate. Is it a complete sentence?
It is considered a complete sentence though the subject is missing because for such sentences in spoken English, the subject is presumed.
The sentence "Make juice' has only a verb (make) and an object (juice) which together make the predicate. The subject is therefore missing.
In written English, this would be an incomplete sentence unless it is used as part of a conversation, in which case, as in spoken English, this would be considered complete since the subject is presumed.
Taking this to be addressed to someone, a likely complete form would be:
"George, you make juice," or just, "You make juice."