I've encountered the same issue, all you need to do is add a line in Player.gd:
onready var skin: = $Mannequinny
I'm confused. My character doesn't move at all with Keyboard input. I didn't get "player.skin" issues. What exactly should the Player and PlayerState script files look like? It's very confusing reading about a solution.
You can download the source code for each chapter here to compare files: https://gdquest.mavenseed.com/lessons/download-all-the-godot-projects-11d7c425d3713e19
In this archive, you have one Godot project per chapter. The projects have a git history with intermediate steps for many of the videos, if you know how to use git.
Your problem shouldn't be related to the issue somebody had in the early access: I updated the videos and code already to fix the bug.
Are you getting any error message? Or it is just that at the end of this lesson, nothing is happening when you press any input? Does the character move with another controller?
If there's no error, it's probably just a function call that's missing or a value that's incorrectly set somewhere.