Yes, you can place a vertical radiator behind a door, provided there is sufficient space to open and close the door without it coming into contact with the radiator.
You might have little choice when installing a radiator in a compact space.
To discover more about the best place to position a new radiator, check out our radiator positioning guide.
And if you would like some inspiration surrounding the best radiators to select, take a look at our comprehensive radiator buying guide. Or, specific to this question, our blog on the Top 5 Vertical Central Heating Radiators.