After deleting my libary countless errors appeared…
I managed to fix most of them by just clicking on the gamescene in my folders instead of starting the project via Unity.
After that i had many issues within the code and even the video’s
manual couldnt help me 
It took me about an hour to realise that “StartGame()” was inside “RunMainMenu()” because of one bracket -.- before this lesson it all worked but then it was too much.
Finally I can go on and maybe I was able to help somebody 
Greetings from Germany