You're starting the game by loading Player.tscn (project settings: application/run/main_scene). However, Player doesn't load anything else. You probably want to load Main.tscn instead.
You still need to change the name of "MobTIimer" to "MobTimer" in Main.tscn.
When you've fixed that, you need to put
add_child(mob)
at the bottom of your _on_MobTimer_timeout() function. The mobs are being created, but you're not adding them to the scene.
