I assume you didn't read https://docs.godotengine.org/en/stable/getting_started/step_by_step/scenes_and_nodes.html which covers every aspects of your question.
MapaDePrueba.tscn
is not a Node
, It's a PackedScene
. The screenshot you showed is your file system, which contains a bunch of Resource
, which haven't been add in your SceneTree
. Scene tree is where the actual game happens in real time.
Also, Godot doesn't care where you put your scripts in the file system. It only cares which node has your scripts.
When you try to get a node, you use node_path
and node.name
. *.tscn is just file name. These below are node names. Node paths are more complex, just read the docs I linked above.

Quick tips:
If you want to get a node from singleton. You first var map = load("res://Scenes/Maps/MapaDePrueba.tscn").instance()
, then add_child(map)
. Then you have it, map
is your node. But in this way, the map is your singleton's child.
If you want to get a node from anywhere, just get_node("/root/path_to_the_node")
, this is the standard way of doing it.
If you want to get a node without knowing the path. just get_tree().get_root().find_node("node_name")
. This is not the best practice, because it's relativey slow.