Network access disabled by default in RetroArch Fedora build due security reasons. Only free libretro cores available which packaged in official Fedora repo. To obtain more libretro cores you need to:
bash
$ retroarch-enable-network-access.shMain Menu -> Load Core -> Download a Core.To reset all settings on defaults and get back to system libretro cores remove user data dir:
$ rm -rf ~/.config/retroarch/