But there is absolute spacetime. Observers will disagree about distance between events in space or time, but they will all agree on the value of the Minkowski metric that includes space and time.
And they'll all disagree on your spatial velocity and how much time has passed for you. The only thing they'll agree on is the spacetime interval between any two events, calculated using the Minkowski metric. That's a fundamental invariant, but it's not absolute spacetime in the sense of a fixed, unchanging background. Instead, it highlights that space and time themselves are relative to the observer's motion.