Overriding Camera.main.ScreenToWorldPoint's z value?

I’m wondering how I would go about this.

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class CController : MonoBehaviour {

    [HideInInspector]
    Vector3 pos;
    [HideInInspector]
    float speed;
    [HideInInspector]
    float jumpheight;
    [HideInInspector]
    float gravity;
    [HideInInspector]
    int isjumping;
    [HideInInspector]
    int canjump;
    [HideInInspector]
    int isfalling;

	void Update () {
        pos = Camera.main.WorldToScreenPoint( this.transform.position);
        
		if ( Input.GetKey(KeyCode.LeftArrow) &! Input.GetKey(KeyCode.RightArrow ))
        {
            pos.x -= 10.0f * Time.deltaTime;
        }
        if (Input.GetKey(KeyCode.RightArrow) & !Input.GetKey(KeyCode.LeftArrow))
        {
            pos.x += 10.0f * Time.deltaTime;
        }
        pos.z = 0;
        this.transform.position = Camera.main.ScreenToWorldPoint(new Vector3(pos.x, pos.y, pos.z));
        
	}
}

The above script is attached to my player sprite. When I run the game in the editor the player is invisible because it adopts the same -10 value as the camera. Even though I put pos.z = 0 the conversion back to World co-ords still reverts back to -10 for the z value.

How do I override this or is there a better way using layers to make my sprite visible?

I am not sure what this script is trying to accomplish exactly to provide more insightful information.
ANyway I did fix your problem, but i did not make any attempt to optimize it

void Update () {
         pos = Camera.main.WorldToScreenPoint( this.transform.position);
         
         if ( Input.GetKey(KeyCode.LeftArrow) &&! Input.GetKey(KeyCode.RightArrow ))
         {
             pos.x -= 10.0f * Time.deltaTime;
         }
         if (Input.GetKey(KeyCode.RightArrow) && !Input.GetKey(KeyCode.LeftArrow))
         {
             pos.x += 10.0f * Time.deltaTime;
         }
         
		 Vector3 temp = Camera.main.ScreenToWorldPoint(new Vector3(pos.x, pos.y, pos.z));
		 pos = temp;
		 pos.z = 0;
         this.transform.position = pos;
         
     }