uos@UOS:~$ lsb_release -r
Release:        20.5
uos@UOS:~$ sudo apt install flatpak
Reading package lists... Done
Building dependency tree      
Reading state information... Done
Package flatpak is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'flatpak' has no installation candidate

От buster:
The following packages have unmet dependencies:
flatpak : Depends: bubblewrap (>= 0.2.1) but it is not installable
           Depends: xdg-dbus-proxy (>= 0.1.0) but it is not installable
           Depends: xdg-desktop-portal (>= 0.10) but it is not installable
           Depends: libappstream-glib8 (>= 0.6.1) but it is not installable
           Recommends: xdg-desktop-portal-gtk but it is not installable or
                       xdg-desktop-portal-backend but it is not installable

История как с Elementary: Красиво, быстро, старые библиотеки несовместимы с апстримом, апстримный реп включишь, всё развалится