object reference is required to access non-static member 'GameManager.CompleteLever()'

I am trying to follow one online tutorial to make a simple game. and I am getting this error object reference is required to access non-static member 'GameManager.CompleteLever()'

using UnityEngine; using System.Collections;

public class PlayerMovement : MonoBehaviour { public float moveSpeed; private Vector3 input; private Vector3 spawn; public float maxSpeed; public GameObject deathParticles; void Start () { spawn = transform.position;

 }
 
 
 void Update () {
     input = new Vector3(Input.GetAxisRaw("Horizontal"),0,Input.GetAxisRaw("Vertical"));
     if (GetComponent<Rigidbody>().velocity.magnitude < maxSpeed)
     {
         GetComponent<Rigidbody>().AddForce(input * moveSpeed);

         
     }
     if (transform.position.y < -3)
     {
         Die();
     }
 }
 void OnCollisionEnter(Collision other)
 {
     if (other.transform.tag == "Enemy")
     {
         Die();
 
     }
 }
 void OnTriggerEnter(Collider other)
 {
     if (other.transform.tag == "Goal")
     {
         GameManager.CompleteLevel();
     }
 }
 void Die()
 {
     Instantiate(deathParticles, transform.position, Quaternion.identity);
     transform.position = spawn;
 }

}