My loading screen won't trigger?

I've been following this tutorial:

https://www.youtube.com/watch?v=xJQXoG3caGc

And I think I have everything working-except the trigger. I can't get the loading screen to trigger while the Play Area of my game is loading, here is a copy of the script in case someone can see where I've gone wrong:

 using System.Collections;
 using UnityEngine.SceneManagement;
 
 public class LoadingScreenManager : MonoBehaviour {
 
     [Header("Loading Visuals")]
     public Image loadingIcon;
     public Image loadingDoneIcon;
     public Text loadingText;
     public Image progressBar;
     public Image fadeOverlay;
 
     [Header("Timing Settings")]
     public float waitOnLoadEnd = 0.25f;
     public float fadeDuration = 0.25f;
 
     [Header("Loading Settings")]
     public LoadSceneMode loadSceneMode = LoadSceneMode.Single;
     public ThreadPriority loadThreadPriority;
 
     [Header("Other")]
     // If loading additive, link to the cameras audio listener, to avoid multiple active audio listeners
     public AudioListener audioListener;
 
     AsyncOperation operation;
     Scene currentScene;
 
     public static int sceneToLoad = -1;
     // IMPORTANT! This is the build index of your loading scene. You need to change this to match your actual scene index
     static int loadingSceneIndex = 4;
 
     public static void LoadScene(int levelNum) {                
         Application.backgroundLoadingPriority = ThreadPriority.High;
         sceneToLoad = levelNum;
         SceneManager.LoadScene(loadingSceneIndex);
     }
 
     void Start() {
         if (sceneToLoad < 0)
             return;
 
         fadeOverlay.gameObject.SetActive(true); // Making sure it's on so that we can crossfade Alpha
         currentScene = SceneManager.GetActiveScene();
         StartCoroutine(LoadAsync(sceneToLoad));
     }
 
     private IEnumerator LoadAsync(int levelNum) {
         ShowLoadingVisuals();
 
         yield return null; 
 
         FadeIn();
         StartOperation(levelNum);
 
         float lastProgress = 0f;
 
         // operation does not auto-activate scene, so it's stuck at 0.9
         while (DoneLoading() == false) {
             yield return null;
 
             if (Mathf.Approximately(operation.progress, lastProgress) == false) {
                 progressBar.fillAmount = operation.progress;
                 lastProgress = operation.progress;
             }
         }
 
         if (loadSceneMode == LoadSceneMode.Additive)
             audioListener.enabled = false;
 
         ShowCompletionVisuals();
 
         yield return new WaitForSeconds(waitOnLoadEnd);
 
         FadeOut();
 
         yield return new WaitForSeconds(fadeDuration);
 
         if (loadSceneMode == LoadSceneMode.Additive)
             SceneManager.UnloadScene(currentScene.name);
         else
             operation.allowSceneActivation = true;
     }
 
     private void StartOperation(int levelNum) {
         Application.backgroundLoadingPriority = loadThreadPriority;
         operation = SceneManager.LoadSceneAsync(levelNum, loadSceneMode);
 
 
         if (loadSceneMode == LoadSceneMode.Single)
             operation.allowSceneActivation = false;
     }
 
     private bool DoneLoading() {
         return (loadSceneMode == LoadSceneMode.Additive && operation.isDone) || (loadSceneMode == LoadSceneMode.Single && operation.progress >= 0.9f); 
     }
 
     void FadeIn() {
         fadeOverlay.CrossFadeAlpha(0, fadeDuration, true);
     }
 
     void FadeOut() {
         fadeOverlay.CrossFadeAlpha(1, fadeDuration, true);
     }
 
     void ShowLoadingVisuals() {
         loadingIcon.gameObject.SetActive(true);
         loadingDoneIcon.gameObject.SetActive(false);
 
         progressBar.fillAmount = 0f;
         loadingText.text = "LOADING...";
     }
 
     void ShowCompletionVisuals() {
         loadingIcon.gameObject.SetActive(false);
         loadingDoneIcon.gameObject.SetActive(true);
 
         progressBar.fillAmount = 1f;
         loadingText.text = "LOADING DONE";
     }
 
 }

I can't get it to trigger but if anyone can help me, I would greatly appreciate it :)