Looking at the error, the reason the game is crashing is because you are trying to instance a scene that does not exist, the mob scene specifically. You will need to break the game up into separate scenes like the tutorial states, at least for the mob scene as otherwise you cannot spawn more than one mob.
You should be able to select the mob RigidBody node you have right now and select something like Save as instanced scene or something similar. Selecting this will allow you to save the node (and it’s children) into its own scene file.
Then all you should need to do to fix the issue is point the preloaded/loaded mob scene in the world script to the scene you just created.
Hopefully this helps!
