Rigidbody.position: How can i make my player to not leave the field

Hi guys! I'm new using unity for a couple of days, and i can't make my player to not leave the playfield on a tutorial "Space shooter - Moving the player"(https://www.youtube.com/watch?v=rVSLczG1M1E&list=PLX2vGYjWbI0RibPF7vixmr4x8ONJX-mNd∈dex=6). When i debug my code, the error that comes is

Error CS1061: 'UnityEngine.Component' does not contain a definition for 'position' and no extension method 'position' accepting a first argument of type 'UnityEngine.Component' could be found (are you missing a using directive or an assembly reference?) (CS1061) (Assembly-CSharp)

Here is my code: using System.Collections.Generic; using UnityEngine;

 public class PlayerController : MonoBehaviour 
 {
     [System.Serializable]
     public class Boundary {
         public float xMin, xMax, zMin, zMax;
     }
 
     private Rigidbody rb;
     public float speed;
     public Boundary boundary;
 
     void Start() 
     {
         GetComponent<Rigidbody>().position = Vector3.zero;
 
     }
 
 
     void FixedUpdate ()
     {
         float speed = 10;
         float moveHorizontal = Input.GetAxis ("Horizontal");
         float moveVertical = Input.GetAxis ("Vertical");
 
         rb = GetComponent<Rigidbody> ();
         Vector3 movement = new Vector3 (moveHorizontal, 0.0f, moveVertical);
         rb = GetComponent<Rigidbody> ();
         rb.velocity = new Vector3 (moveHorizontal * speed, 0.0F, moveVertical * speed);
 
         Vector3.zero = new Vector3 (
             Mathf.Clamp (rigidbody.position.x, boundary.xMin, boundary.xMax),
             0.0f,
             Mathf.Clamp (rigidbody.position.z, boundary.zMin, boundary.zMax)
         );
 
     }
 }

Any help? Ps: Sorry for my english, like my unity skills, i'm working on it, so if you see my english bugs, tell me.