No, you can ask about a movie without having seen it.
I speak from personal experience, since I haven't seen Star Wars. (Different site, but the principle should be the same.)
But be careful what kinds of question you ask, or you may end up embarrassing yourself.
Continuing with the example of Star Wars: if you haven't watched the films and you ask "who is Luke's father?", you're likely to be downvoted into oblivion for lack of research. Nobody who has seen the films is going to be remotely interested in the answer to that question. But if you ask an out-of-universe question like "why was this character killed off?" or "how did they achieve this effect?", that's a safer bet. Watching the films doesn't usually tell you anything about behind-the-scenes stuff, so you don't have much of a disadvantage for these questions compared to someone who has watched them.
A good rule of thumb might be to ask yourself:
Will this question and its answers be interesting even to someone who has seen the movie?
- If the answer is no, don't post it: most of the people following the tag for a given movie will be people who've seen it, and if none of them are interested, your question is unlikely to fare well.
- If the answer is yes, by all means post it. A good class of questions which you can ask about a movie without having seen it are out-of-universe or behind-the-scenes ones, as said above.
- If you don't know, then it's a gamble. Without having seen the film, it may be impossible for you to tell whether, from the point of view of those who have, your question is trivial or interesting. You can still post it, but don't be too surprised if you end up getting boatloads of downvotes because the answer to the above question turned out to be no.