How does one make a goalpost?

I am making a my first ever game, and need to know how to make a goalpost area.

Essentially, the trigger to spawn a next level. I am using 3 different scripts that should work together, but dont GameManager, EndTrigger, and Level Complete. I also have a UI for when a level's beaten that the button wont work on. any advice is appreciated.

the code to those who need to see it: GameManager:

 bool gameHasEnded = false;

 public float restartDelay = 1f;

 public GameObject completeLevelUI;


 public void CompleteLevel ()
 {
     completeLevelUI.SetActive(true);
     
 }

 public void EndGame ()
 {
     if (gameHasEnded == false)
     {
         gameHasEnded = true;
         Debug.Log("GAME OVER");
         Invoke("Restart", restartDelay);
     }
 }

 void Restart ()
 {
     SceneManager.LoadScene(SceneManager.GetActiveScene().name);
 }

This is the LvlComplete:

 public void LoadNextLevel ()
 {
     SceneManager.LoadScene(SceneManager.GetActiveScene().buildIndex + 1);
 }

}

And the EndTrigger:

 public GameManager gameManager;

 void OnTriggerEnter ()
 {
     gameManager.CompleteLevel();
 }

}