Does my novel have to be diverse?

Have to? Of course not. Should? Perhaps so. If all your characters, major and minor, are white people in a distant galaxy in the future, does that not suggest that only white people were chosen/were able/could afford/were smart enough to leave Earth to come there? Does this say something negative?

To me, it does.

Not that you have to make diversity an element of your story. If you opted to have a more diverse cast, it doesn't even have to be an element. Mere existence would be sufficient, the same way an author might place a gay or disabled character in a work of fiction that's got nothing to do with anybody's sexual orientation or physical ability.

Consider the probable ways other-world colonists are likely to be selected. I would think either race would not be a factor, or that diversity would be deliberate. Of course, in successive generations the diverse races and their original cultures taken off-planet would blend, perhaps into a single culture, but it won't be that of white men and women.

You are, of course, free to write this as you choose. If it were my work, I'd mention appearance in ways that suggest not everybody is a direct descendant of a Caucasian line, without making it at all a big deal.

I think a good question to ask yourself is why in this futuristic universe, are the people of your world all white? Is that a realistic expectation based on the history of getting humans to this planet? Was it a choice made by the people who left Earth and colonized this particular planet? Was there a scientific or cultural reason that would have led to this particular make-up of races (or lack thereof)? It's a sci-fi. You don't have to know all the science or even the history, but when the questions come up, you should be able to come up with an answer or change it.

Does your novel have to be diverse? No it doesn't. But when your setting is important, it's worth examining whether the make-up of the characters is realistic for that setting. That goes for speculative fiction, contemporary fiction, historical fiction, etc.

Just because in your futuristic setting you have characters of different races doesn't mean that race will be an issue. That far into the future, you get to make up how much of an issue it will be. You look at Star Trek: Deep Space Nine, and having a black captain wasn't a big deal for anyone of the time period...but when they had to deal with Earth's history (holodecks, time travel, dreams, whatever), they addressed the issue because it was one for those time periods. Sisko was aware of the history, but it didn't affect how he interacted with his crew or how they interacted with him.

IMO, having no diversity within a futuristic cast makes race more of an issue than making it diverse without the modern tensions. It means that at some point you will have to address why all the characters are all white (straight, cis, abled, etc.).

You can always take the route of shrugging and claiming that you never assigned race so it's the reader's fault that they read them as white, but just like you have defaulted to white, so will many of your readers, because it is the "default" if there are no descriptions to draw from. And we know from this thread that it's backpedaling.

I'm not a physical description person, but I do like occasional features assigned to characters. It's surprising if your POV character never notices what anybody looks like, unless they're blind or have prosopagnosia.

Nobody's asking you to make it a theme, nor to make a token character of each race. What I see people asking is to address your question from a world-building standpoint. You see this cast as white. Is that realistic for the world you're building and why?