Time machines, though fictional, could be made, but would not be practical.
Let’s say you have one that actually works.
You program it to go back or forward in time by one second.
It works.
You instantly die.
Why?
Because the Universe and everything in it is in motion.
The Earth is rotating on its axis, and is revolving around the Sun.
The Sun and all its retinue of planets, various moons, asteroids, comets and dust are in motion orbiting around the Milky Way Galaxy’s black hole at its center.
The galaxy it self is in motion in some direction away from the ‘center’ of the Universe.
The entire Universe is expanding.
So in that ONE SECOND, forward or backward, you wind up in empty space! Or, if you’re very UNLUCKY the middle of a star somewhere and are instantly vaporized.
So, time travel may be possible, but would be quite unpractical...............
You do know, of course, that you are a time machine.
You just move forward in one direction—very slowly.
;-)
Time travel depends on everything already existing at the same time—past, present, future—and then the machine is just a way to move around....
The concept is like a reel of film.
The film is already made—but if you were a character in the film you just play your part—the time traveler can see any part of the film at any time, in any order.
Whether that is actually how the universe works—I have no clue—but imho the critics of time travel have no clue either.