can't Jump Properly,can't Jump Properly, my first time animate

I dont know what to do, when I click play, I click Space for Jump and sometime it jump but sometime it not. What do I do wrong?

i got it from https://www.youtube.com/watch?v=I_mjYhwSsS8

this is my PlayerController Script.

 private void FixedUpdate()
 {
     Grounded();
     Jump();
     Move();
 }

 private void Jump()
 {
     if(Input.GetButtonDown(Jump) && this.grounded)
     {
         this.rb.AddForce(Vector3.up * 4, ForceMode.Impulse);
     }
 }

 private void Grounded()
 {
     if(Physics.CheckSphere(this.transform.position + Vector3.down, 0.2f, layerMask))
     {
         this.grounded = true;
     }
     else
     {
         this.grounded = false;
     }
     this.anim.SetBool("jump", !this.grounded);
 }