So I'm pretty new to the Unity Engine as well as scripting in C#, but I need help with figuring out how to make a canvas appear when my player leaves the camera screen.

sing UnityEngine;
using UnityEngine.UI;
using System.Collections;

public class PlayeyLose : MonoBehaviour
{
public Canvas gameOver;

void Start () 
{
	gameOver = GetComponent<Canvas> ();
	gameOver.enabled = false;
}

void OnBecameInvisible()
{
	Time.timeScale = 0f;
	gameOver.enabled = true;
}

}

edit your codeā€¦

public class PlayeyLose : MonoBehaviour {

public Canvas gameOver;

 void Start () 
 {
    
     gameOver.enabled = false;
 }
 
 void OnBecameInvisible()
 {
     Time.timeScale = 0f;
     gameOver.enabled = true;
 }
}

assign your canvas to gameover object in inspector.