When you install or upgrade to Fedora Workstation 35, if you don’t opt-in to Fedora Third-party repositories initially (during the first boot after the installation, or from an info bar in GNOME Software after an upgrade), you are supposed to be able to opt-in through the GNOME Software repositories dialog later. However, the “Fedora Flathub Selection” third-party repository will not be visible there, so cannot be enabled.
Note: This problem only affects Fedora 35 and Fedora 36 Beta. It is resolved for fresh installs of Fedora 36 final release.
Bugzilla report: #2011274
As a workaround for already installed systems, run this from the command line:
sudo fedora-third-party disable sudo fedora-third-party enable
This will result in all third-party repositories being created and enabled. You can disable any repositories you don’t want through the GNOME Software repositories dialog.
This problem has been fixed for Fedora 36 final (not Beta) install images, so cleanly installed Fedora 36 systems will not be affected.