Actually, I downloaded your project. When I play the “Main” scene your whole game works fine.
The problem is that you have set the “Player” scene set as the scene that runs when you press the “run” button. Open “Project Settings” and change this in the “Application/Run” section.
I got the same problem when I run the debug in the Godot editor. I can see an empty scene. Then I max size the game window and the game contents come out.
Finally, I check the project settings->display->window->fullscreen on, and it works.