Hi, Mike! You need to instantiate the scene first and then add it to the tree. It should be something like:
func spawn_ghost(): var ghost_instance = ghost.instantiate() add_child(ghost_instance)
Also, note that you're using add_child($"."), and the dot notation is used to reference the node attached to the script, it's like 'self'. In this case, the dot is making reference to de node with ghosts_spawn.gd attached to it.
Hope I helped!