There's no x,y, or z axis independent of the observer's frame of reference either. Does that mean that spatial travel is not a coherent concept?

If the coherent concept is 'spacetime travel', why is it required that there exist an ordering over all points in spacetime? Every pair of points (A,B) falls into one of three categories: events at point A can directly or indirectly have an effect/be observed at point B, but not vice versa; events at point A cannot have an effect or be observed at point B, directly or indirectly; events at point B can be observed or have an effect at point A, directly or indirectly.

It is difficult for different observers to communicate where points are, but they divide all points in spacetime into the same three categories.