You can type sudo apt install snapd to install snap and sudo apt install flatpak to install flatpak. After installed flatpak, you need to relogin to apply XDG_DATA_DIRS. Also, you need remote-add a flatpak repo before you search or install a flatpak app.

Flatpak：https://flatpak.org/setup/Deepin

Snap: https://snapcraft.io/docs/installing-snap-on-debian

https://snapcraft.io/snap-store

Unfortunately, I also can't get it installed.
The explanation on flatpack.org doesn't work.