The theory of relativity closes off FTL travel and must do so to preserve causality.

In relativity, we assume that the speed of light in the vacuum (which, in the mathematical and physical sense also means there isn't anything for it to be relative to) is the same in every frame of reference. This is what Michelson–Morley experiment measured, and it is remarkable. It is unintuitive, because if you were throwing someone a ball at rest vs from a moving car, the moving one would have a different velocity. But that doesn't happen like that with light. Regardless if you're stationary or moving away or toward me at 0.9c, we'll both see both c as c.

What we can't do is accelerate things to the speed of light if they were sub-luminal for the same reason we can't decelerate super-luminal objects to the speed of light: it requires infinite energy. By the way, doing that to light in either direction is equally difficult and for the same reason. This, however, doesn't mean that FTL travel is impossible. To be precise, relativity describes it. It doesn't describe how to jump over the light speed barrier, not how to treat the light speed as some unbreachable limit of the universe. Why FTL could happen and how it relates to mass? Where do I even start, Higgs fields? QCD binding energy? Affine spaces? Maybe, but I doubt you'd read it.

No, I'll just restate this one more time: it doesn't have to forbid or preserve a thing. It can't, on its own, explain how we go from sub-luminal to light speed or super-luminal down to light speed, or how to jump over the speed of light in either direction, but it doesn't prohibit faster than light travel. It literally describes time travel in both directions, not how to move over discontinuities. Which, mind you, could be purely mathematical artefacts.

I'll give you one better: the problem with wormholes isn't that we can't figure out negative energy or transit stability, it's that we wouldn't be able to make them connect specific points in spacetime. It's for a similar reason why you wouldn't be able to steer Alcubierre bubble as we understand it, not why they couldn't exist.