I recently had an issue with scene loading through scripts and found out that it was related to the usage of strings and plain text settings files:
Scene 'Scene_Game' couldn't be loaded because it has not been added to the build settings or the AssetBundle has not been loaded.
To add a scene to the build settings use the menu File->Build Settings...
Scene 'Scene_Game' couldn't be loaded because it has not been added to the build settings or the AssetBundle has not been loaded.
To add a scene to the build settings use the menu File->Build Settings...
As you can see from above, 'Scene_Game' is registered in the editor. So what was the deal? There is a plain text file located in [ProjectRoot]\ProjectSettings\ called 'EditorBuildSettings.asset' that may need to be updated with the modified scene file name.
This error occurred for me because I renamed the scene file within the assets browser in engine, and it failed to also update the file in the file system.
This error occurred for me because I renamed the scene file within the assets browser in engine, and it failed to also update the file in the file system.