You are not missing a scene, you are missing something called namespace at the top of your code.
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.SceneManagement; // This is the missing line.
About the canvas, not sure what you meant. A screenshot of your screen and the time and lesson you are on to compare would be great to help you further.
I’ve been on this forums for quite some time and I’ve seen people that could do a game after finishing the first two sections of the course, but I’ve also seen the complete opposite, people that have done not just the 2D and 3D course, but other courses in other sites, yet they still struggle quite a bit with coding. What’s the difference? I’m not quite sure to be honest, I suppose it has something to do with personality, but I don’t want to write an essay about that here.
To make progress, just keep trying and do all the challenges, ask as many questions as you can, take advantage of this forums, and also, experiment a bit, don’t be afraid to break things.
Ok, I just saw Rick’s Canvas and it is indeed enabled, I’m not 100% as to why is that, it might have something to do with which Unity version he’s using, but don’t worry too much about that, he doesn’t modify it directly, he does modify the childed objects, but those will be enabled.