This example requires the .NET version of Godot to work. It will not work with the standard version of Godot. You can download the .NET version of Godot from https://godotengine.org/download. Note that the Steam version of Godot is the standard version and will not work with this example. If you just freshly imported this into Godot and have not yet added other C# files to the project, you will need to initialize the project for C# by going to Project -> Tools -> C# -> Create C# Solution. Otherwise the demo will not work because its files have not been compiled.