You are probably trying to save into the APK, which will not work. What path are you saving to?
You could use "/Storage/config.json" for example, but "config.json" will not work in an APK because files in the project folder are stored in a read-only APK when installed to the device. The. "/Storage/.." path format will access the storage area of your app (on both scoped and non-scoped) devices.