You need to give Godot a chance to fire up with:
Change var scene = load("res://scenes/tile.tscn")
to var scene = preload("res://scenes/tile.tscn")
Move the rest of the code within your _ready
method. As the default comments say, that fires when your node is good to go. In Godot, you can only declare member variables outside methods, everything else should go in a function. Like this:
extends Node2D
var scene = preload("res://scenes/tile.tscn")
func _ready():
var scene_instance = scene.instance()
add_child(scene_instance)
ps. You see the little curly braces above when you type? Just highlight the code and click that, it'll be a million times more readable.