The get_node() function needs a node path based on the layout of the nodes in the scene, not in your res:// directory. Look out how the nodes are laid out in the “Scene” tab, not the “FileSystem” tab.
For example, if your scene had a node hierarchy like this:
what is your node structure? if you have both nodes as siblings in the node tree you can access the root node of the scene and then reference the player node