So there's a shitload of codes out there that don't work. Obviously, there's a lot of them.
This is the simplest explanation I found (surprise surprise) and you don't have to listen to a 12yr old on youtube.
I got bloody mad at videos etc cause they didn't work (because the codes are different for different manufacturing runs, or whatever.
I registered the xbox with the serial that was in the menu - it kept not recognising the one on the back of the xbox. Why are they different? Idfk. But here's the code that worked for me. Enjoy.