Did you give it enough time to load them? Is it possible your system has some security block?
To compare and contrast in a place where it works…
If you browse to here and click on
launch binder, presently you get version 1.2.6 like you are using.
Next if you click on
Enable Extensions Manager,
Enable and then do what you show in your screenshot, after some time, you’ll see the extensions listed. It takes quite a bit of time for the list to load and update. Both the first ones and then the ones under ‘Discover’. (I toggled the ‘Discover’ arrow closed and open, and then it loaded. However, I don’t know if that was coincidence and it just had finally finished updating.)