Cannot cremate corpses: "Cannot haul *. No empty, accessible spot"

It turns out the problem was that, like crafting, you right click on the station (the crematorium), not the "material" (a corpse).

If I right click on the crematorium with a colonist and click "Prioritize cremating at Granite electric crematorium", they will cremate a nearby corpse (although you don't seem to be able to pick which one).

The crematorium (and other workbenches) require you to have the items stored somewhere in your home area. If you don't have any items stored there, a colonist working with the bench can still go and fetch suitable items from the rest of the map though - in fact they will prioritize such items.

So what you would typically do is to create a zone near the crematorium such as a dumping stockpile that only accept human corpses. Then one colonist can burn corpses while others haul. If you have some psycho or bloodlust colonists, they are usually preferred for this kind of work since they get no mood penalty from dealing with dead bodies.