Debian Install Flatpack

1) 

sudo nano /etc/apt/sources.list

2) 

Just copy/paste the 3 following lines into your ( sudo nano /etc/apt/sources.list ) :

deb https://deb.debian.org/debian/ bookworm main contrib non-free non-free-firmware
deb https://deb.debian.org/debian bookworm-updates main contrib non-free non-free-firmware
deb https://security.debian.org/debian-security/ bookworm-security main contrib non-free non-free-firmware

(https://forums.debian.net/viewtopic.php?p=788514&sid=76c03f66aa66f7f82536fa72789a9a5e#p788514)

3) 

Follow https://flathub.org/setup/Debian