It’s not theoretically impossible, it’s just really hard. If we go fast enough, we can even do it within the current lifespan of a human (from the perspective of that human).
I meant it's theoretically impossible with the theory that we have now. Of course if we make breakthroughs in the theory, then the theory will change and it may become possible.
But SpaceX going to Mars is not exactly revisiting this theory. They are just having fun with the current state of the theory, which says that they won't bring humans further, period.
Relativistic speeds require too much energy, too much reaction mass, and too much fuel. None of the reaction drives are good enough. It would only be possible with magic reactionless drive, and magic source of power.