To summarize installing Chirp using flatpak in Debian:

If you are using GNOME Desktop environment, install the Software Flatpak plugin:

[as root:] apt update | apt install gnome-software-plugin-flatpak

Non-GNOME Desktop Environments:

For Debian Buster and newer (including Bullseye and Sid):
A flatpak package is available. To install it, run the following as root:

apt install flatpak
   
For Debian Jessie and Stretch:
A flatpak package is available in the official backports repository;
Too add this repository (if it's not alreay there):

Add using root to your /etc/apt/sources.list file this line:
deb http://deb.debian.org/debian buster-backports main

Now update and install flatpak:

[as root:] apt update | apt upgrade | apt install flatpak

Now you have to REBOOT or at least log out and back in.    

You have to download the Chirp flatpak from the Chirp download page and then install with

So do this:

Go to https://trac.chirp.danplanet.com/chirp_daily/LATEST/

Find the latest which of today is:
https://trac.chirp.danplanet.com/chirp_daily/LATEST/chirp-daily-20210830.flatpak

Save that to ~/Downloads, then:

cd ~/Downloads

sudo flatpak install chirp-daily-20210830.flatpak

And everything works.

Thanks Nigel,

73
DR
N1EA