how do I make it so that when I left click my character will knock backward?

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

public class shotgunKnockback : MonoBehaviour
{
// Start is called before the first frame update
public CharacterController controller;
public float speed = 12f;

void Start()
{

}

    // Update is called once per frame
void Update()
{
    if (Input.GetMouseButtonDown(0))
    {
        float x = Input.GetAxis("Horizontal");
        float z = Input.GetAxis("Vertical");

        Vector3 move = (transform.right * x + transform.forward * z) * -500f;

        controller.Move(move * speed * Time.deltaTime);

    }
}

}

It looks like you are trying to also handle character run movement too? Here’s an example of how you would do both:

public class shotgunKnockback : MonoBehaviour { 
    public CharacterController controller; 
    public float speed = 12f;

    //how far back the knockback goes
    public float knockbackDistance = 0.5f;

    void Update()
    {
        // get player inputs
        float x = Input.GetAxis("Horizontal");
        float z = Input.GetAxis("Vertical");

        // multiply player inputs by speed, and deltaTime, to get distance to move this frame.
        // store that in "move"
        Vector3 move = (transform.right * x + transform.forward * z) * speed * Time.deltaTime;

        //if we clicked
        if (Input.GetMouseButtonDown(0))
        {
            // multiply the knockback by the backward direction, and add that to our "move" variable
            move += -transform.forward * knockbackDistance
        }

        // move the character by the stored amount
        controller.Move(move);
    }
}

If you only wanted the knockback, and no walking movement, all you need is this:

public class shotgunKnockback : MonoBehaviour { 
    public CharacterController controller; 

    //how far back the knockback goes
    public float knockbackDistance = 0.5f;

    void Update()
    {
        //if we clicked
        if (Input.GetMouseButtonDown(0))
        {
            // multiply the knockback by the backward direction, and move
            controller.Move(-transform.forward * knockbackDistance)
        }
    }
}