URP stops a shader from working

I am making a game, and I have a pixelation shader and script. Everything works how it is supposed to, until I turn on URP. I don't know why, but the pixelation just stops as soon as I set it to the render pipeline.

Is there a reason for this? Have I configured the URP incorrectly, or is it just a problem with the shader/script? If it is a problem with the shader/script, what can I do to fix it.

(I downloaded the pixelation shader and scrpt from the web, so I don't exactly know how they work.)

Let me know if you need the shader or script.