Why are Assets disappearing in game

Godot Version

4.2

Question

Hello,
i am trying to programm a 3D game in Godot for a school projekt, but dont have any experience with Godot or Unity in general. Rn I am facing a Problem that Assets that i loaded in are showing, but when i start the game they disappear. I dont know how to fix it. Also i created a Blender Character and used some animations from the kaykit, But now his head seems to be facing in the wrong direction and i cannot turn it around since everytime i go in the animation player the head faces the wrong direction again.
thank you for any kind of help:D