The extension manager only supports installing extensions for versions of Slicer corresponding to the revisions of Slicer Preview builds. The latest Slicer Preview 4.13.0-2021-09-18 is revision 30223, so extensions from the factory server are available for that version but not for the revisions of any commits from today. If you have your own Slicer that you’ve manually built, you are expected to manually build any extensions you need as well.