The Flatpak app is included as part of the desktop bundle. Make sure the desktop bundle is installed before installing a Flatpak app: sudo swupd bundle-list | grep desktop

To install the desktop bundle, enter: sudo swupd bundle-add desktop

To add this Flatpak app, enter: flatpak install flathub net.sourceforge.ExtremeTuxRacer

To run this Flatpak app, enter: flatpak run net.sourceforge.ExtremeTuxRacer

For additional Flatpak commands, enter: flatpak --help