What is wrong with this code for adjusting the walls and players to match the screen size?

Hey, I'm working on a pong clone from a tutorial series (it's in java while I'm coding in c# so I'm running into more errors trying to translate things) and I've got an error with this code for my game setup, though I'm not sure what it is.

The code is supposed to adjust the size of the wall colliders to the edges of the screen and adjust the position of the player's bars accordingly. (I'll figure out how to scale them later)

The error says:

And when I double click on them it takes me to the last two lines of my code. All of the code is here:

 using UnityEngine;
 using System.Collections;
 
 public class gameSetup : MonoBehaviour {
 
     public Camera mainCam;
 
     public BoxCollider2D topWall;
     public BoxCollider2D bottomWall;
     public BoxCollider2D leftWall;
     public BoxCollider2D rightWall;
 
     public Transform player01;
     public Transform player02;
 
     public float pixFromEdge = 75;
 
     void Start () {
         //Move each wall to it's edge location
         topWall.size = new Vector2 (mainCam.ScreenToWorldPoint (new Vector3 (Screen.width * 2f, 0f, 0f)).x, 1f);
         topWall.center = new Vector2 (0f, mainCam.ScreenToWorldPoint (new Vector3 (0f, Screen.height, 0f)).y + 0.5f);
 
         bottomWall.size = new Vector2 (mainCam.ScreenToWorldPoint (new Vector3 (Screen.width * 2f, 0f, 0f)).x, 1f);
         bottomWall.center = new Vector2 (0f, mainCam.ScreenToWorldPoint (new Vector3 (0f, 0f, 0f)).y - 0.5f);
 
         leftWall.size = new Vector2 (1f, mainCam.ScreenToWorldPoint (new Vector3 (Screen.height * 2f, 0f, 0f)).y);
         leftWall.center = new Vector2 (mainCam.ScreenToWorldPoint (new Vector3 (0f, 0f, 0f)).x - 0.5f, 0f);
 
         rightWall.size = new Vector2 (1f, mainCam.ScreenToWorldPoint (new Vector3 (Screen.height * 2f, 0f, 0f)).y);
         rightWall.center = new Vector2 (mainCam.ScreenToWorldPoint (new Vector3 (Screen.width, 0f, 0f)).x + 0.5f, 0f);
 
         player01.position.x = mainCam.ScreenToWorldPoint (new Vector3 (pixFromEdge, 0f, 0f)).x;
         player02.position.x = mainCam.ScreenToWorldPoint (new Vector3 (Screen.width - pixFromEdge, 0f, 0f)).x;
     
     }
 }
 

I don't understand exactly what the error message is talking about.

I'll appreciate any help.