Can someone help me with the jumping in this Player Controller script?

This script lets me jump once in my 2D platformer, but after that I can't jump again. Why isn't the collision making canJump equal true? Here is my entire Player Controller script:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.Sprites;
 
 public class PlayerController : MonoBehaviour 
 {
     public float speed;
     public float jumpSpeed;
     public Rigidbody2D rb;
 
     private Vector3 movement;
     private bool canJump=true;
 
     void Start ()
     {
         rb = GetComponent<Rigidbody2D>();
         bool canJump = true;
     }
 
     void Update () 
     {
         transform.rotation = new Quaternion (0.0f, 0.0f, 0.0f, 0.0f);
         if (Input.GetKey (KeyCode.A)) 
         {
             transform.Translate (-Vector3.right * Time.deltaTime * speed);
         }
 
         if (Input.GetKey (KeyCode.D))
         {
             transform.Translate (Vector3.right * Time.deltaTime * speed);
         }
 
         if (Input.GetKeyDown (KeyCode.Space)&&canJump) 
         {
             rb.AddForce (transform.up * jumpSpeed);
             canJump=false;
         }
     }
     void OnCollision(Collision col) 
     {
         if (col.gameObject.tag == "Ground") 
         {
             canJump=true;
         }
     }
 }