Rear black plastic bumper

You also have scratches on the lower colored panel below the black bumper, and I can see a bunch of scratches on the cargo step.

The black plastic, as far as I know, cannot be polished or buffed smooth. It is the nature of the plastic.

You could mirco-sand it to smooth out the worst of the scratches, but that will take time.. and a range of finer and finer sand paper to do so. But I personally would not so.. unless I was prepared to evenly sand the entire black bumper section with the same finish and appearance.

At the end of the day.. the black plastic on the bumper is MEANT to take the damage that you have experienced.