How to get a rigidbody to move left and right?

 using UnityEngine;
 using System.Collections;
 
 public class LeftAndRightTest : MonoBehaviour {
 
     public float speed = 6.0F;
     private Vector2 moveDirection = Vector2.zero;
     void Update() {
         CharacterController controller = GetComponent<CharacterController>();
         if (controller.isGrounded) {
             moveDirection = new Vector2(Input.GetAxis("Horizontal"), 0);
             moveDirection = transform.TransformDirection(moveDirection);
             moveDirection *= speed;
             
         }
         controller.Move(moveDirection * Time.deltaTime);
     }
 }

This should work. No guarantees. This does require a character controller on the object that the script is on.