Replace shader at runtime

I have looked around in the docs on how to change all shaders in the scene during runtime and i wrote this and attached to my main camera:

 using UnityEngine;
 using System.Collections;
 
 public class ReplaceShader : MonoBehaviour {
 
     public Shader grayScale;
 
     void Update() 
     {
         Camera.main.SetReplacementShader(grayScale, "Grayscale");
     }
 }

When i try and run the game all that shows up is the default "blue background", the game is still running in the background but you can't see it through the camera, how could i fix this?

(Note I am beginner to coding and to unity)

-------------------------------------EDIT-------------------------------------------------

I found a solutions that kind of works, i changed this line:

 Camera.main.SetReplacementShader(grayScale, "Grayscale");

to this:

 Camera.main.SetReplacementShader ( grayScale, "RenderType" );

Now the camera shows the grayscale, but for some reason the shader is really dark? :/