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.name. *.tscn is just file name. These below are node names. Node paths are more complex, just read the docs I linked above.
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.