I have no problem loading it from the menu and no problem loading it from terminal. In terminal, it does indeed complain about the kns-backend, but this is a red herring as it still works.
Are you sure your installation was made with valid installation media? Like did you check the integrity as the manual states?
I did sha256sum the .iso file and used the ‘Startup Disk Creator’ to flash a usb drive, I used this same drive on my other computer and Discover works fine on it.
Is it possible to purge/re-install plasma-discover?