Ubuntu Install for flatpak doesn't include runtime by default
error: The Application org.pitivi.Pitivi/x86_64/stable requires the runtime org.gnome.Platform/x86_64/3.24 which is not installed
http://docs.flatpak.org/en/latest/dependencies.html#runtimes
flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
flatpak install flathub org.gnome.Platform//3.24