Double jumping not working - can't find the culprit

Hello

I'm new to scripting and thus follow a tutorial. My double jumping isn't working, though, and I went through the code in the tutorial again, but couldn't find the culprit. I tried adding a public Rigidbody2D to reference and then set it's velocity on the Y axis to 0, but alas. Here's my entire player script, sorry if it looks a bit weird and confusing, I took the irrelevant parts out:

 using UnityEngine;
 using UnityEngine.UI;
 using System.Collections;
 using UnityEngine.SceneManagement;
 
 public class Player : MonoBehaviour 
 {
     public float power = 500;
     public int jumpHeight = 1000;
     public bool isFalling = false;
     public int Score;
     public Text SCORE;
     public GameObject YOUDIED;
 
     private int health = 3;
 
     public GameObject health1;
     public GameObject health2;
     public GameObject health3;
     public int Highscore;
 
     private bool canDoubleJump;
     private bool jumpOne;
     private bool jumpTwo;
 
     // Use this for initialization
     void Start () 
     {
         YOUDIED.SetActive (false);
         Highscore = PlayerPrefs.GetInt ("Længste karriere: ", 0);
         jumpOne = false;
         jumpTwo = false;
         canDoubleJump = false;
 
     }
 
     void Update()
     {
         if (Input.GetKey (KeyCode.Space) && health == 0)
         {
             Time.timeScale = 1f;
             health = 3;
             SceneManager.LoadScene (0);
         }
         if (health == 3) 
 
         {
             health1.SetActive (true);
             health2.SetActive (true);
             health3.SetActive (true);
         }
 
         if (health == 2) 
         {
             health1.SetActive (true);
             health2.SetActive (true);
             health3.SetActive (false);
         }
         if (health == 1) 
 
         {
             health1.SetActive (true);
             health2.SetActive (false);
             health3.SetActive (false);
         }
         if (health == 0) 
 
         {
             health1.SetActive (false);
             health2.SetActive (false);
             health3.SetActive (false);
         }
         if (health <= 0)
         {
             YOUDIED.SetActive (true);
             Time.timeScale = 0.0f;
         }
         SCORE.text = "Dage " + Score;
         transform.Translate (Vector2.right * power * Time.deltaTime);
         if (Input.GetKeyDown (KeyCode.Space) && isFalling == false) 
         {
             jumpOne = true;
             canDoubleJump = true;
             isFalling = true;
         }
         if (Input.GetKeyDown (KeyCode.Space) && isFalling == true && canDoubleJump == true) 
         {
             jumpTwo = true;
             canDoubleJump = false;
         }
     }
     
     // Update is called once per frame
     void FixedUpdate () 
     {
         if (jumpOne == true) 
         {
             GetComponent<Rigidbody2D> ().AddForce (Vector2.up * jumpHeight);
             jumpOne = false;
         }
         if (jumpTwo == true) 
         {
             GetComponent<Rigidbody2D> ().AddForce (Vector2.up * jumpHeight);
             jumpTwo = false;
         }
     }
     void OnCollisionStay2D (Collision2D coll)
     {
         if (coll.gameObject.tag == "Ground") 
         {
             isFalling = false;
             canDoubleJump = false;
         }
 
     }
     public void ScorePlus (int NewScore)
     {
         Score += NewScore;
         if (Score >= Highscore)
         {
             Highscore = Score;
             PlayerPrefs.SetInt("Længste karriere: ", Highscore);
         }
     }
 
     void OnTriggerEnter2D (Collider2D coll)
     {
         if (coll.gameObject.tag == "Death")
         {
             health -= 1;
     }
  }
 }