Lock Character to Z axis.

I have a basic script that allows me to move on the X and Y(jump). I want to run a check that the character is always on a certain Z axis. I have tried the transform.position.z = 20; but that gives me a "Cannot modify type value" error.

Here is the code in using (similar to CharacterController.Move sample code)

 using UnityEngine;
 using System.Collections;
 
 public class characterController2D : MonoBehaviour {
     
     public float speed = 6.0F;
     public float jumpSpeed = 8.0F;
     public float gravity = 20.0F;
     
     private Vector3 moveDirection = Vector3.zero;
     
     void Update() {
     
         CharacterController controller = GetComponent<CharacterController>();
         
         
         
         // If character is on the ground
         if (controller.isGrounded) {
             moveDirection = new Vector3(Input.GetAxis("Horizontal"), 0, 0);
             moveDirection = transform.TransformDirection(moveDirection);
             moveDirection *= speed;
             
             //Jump from grounded position
             if (Input.GetButtonDown("Fire1")){
                 moveDirection.y = jumpSpeed;    
             }
         }
         
         if(!controller.isGrounded){
             moveDirection.x = Input.GetAxis ("Horizontal");
             moveDirection.x *= speed;
         }
             
         moveDirection.y -= gravity * Time.deltaTime; //
         print (moveDirection.z);
         controller.Move(moveDirection * Time.deltaTime); //
     }
 
 }