That "folder" is a node type that is parent to other node types. The reason it can't be instantiated is that it is abstract. As far as I know GDscript does not support abstract classes, so you'll have to either create a non-abstract base class or add the abstract base class from the C++ backend (which would require a custom module or a PR that officially adds it to the engine).
In any case you should be sure that a base class is what you need. If you think of it as a "folder" you may be on the wrong track.