Since I don't know your operating environment or the purpose of your implementation, I may be misguided.
First, please make sure that the file is saved successfully.
Specifically, check the return value of the ResourceSaver.save function to see if OK is returned.
If not, try changing the location where the file is saved.
In my environment, ResourceSaver and ResourceLoader did not work as expected when the "user://" prefix was given.
If it returns something other than null, then the loading is done, but some loaded files may not be usable as-is.
In this case, it is tscn, so you need to change it to instance.
Reference Resource)
https://docs.godotengine.org/en/stable/getting_started/step_by_step/resources.html
If your goal is to save an arbitrary string, you can write it directly to a file.
If the structure is complex, you can read and write it in JSON format.
Reference File, JSON)
https://docs.godotengine.org/en/stable/classes/class_file.html
https://docs.godotengine.org/en/stable/classes/class_json.html