Seven days ago I posted this link (that needs to be understood):
Over there is recommended simple (very kind) method that I didn’t test with PureOS Amber, meaning it is/was up to you: echo "deb http://download.opensuse.org/repositories/home:/rpavlik:/bullseye-fix/Debian_Testing/ ./" | sudo tee /etc/apt/sources.list.d/bullseye-upgrade-fix.list
curl http://download.opensuse.org/repositories/home:/rpavlik:/bullseye-fix/Debian_Testing/Release.key | sudo tee /etc/apt/trusted.gpg.d/bullseye-upgrade-fix.asc
sudo apt update && sudo apt upgrade
Otherwise just download PureOS 10.0 image, as of 10-Oct-2021, install it and reread my post #17 here.
Yes, it is always useful to do that. Afterwards you’ll find out that you need, I think:
As already posted by yourself go (as first step) with that one or some local variation of it: deb https://ftp-stud.hs-esslingen.de/debian/ bullseye non-free
@Ralf, afterwards please just follow things (recommendations) from my post #17 here (don’t move to the next step if previous isn’t working smoothly), start with (just to be aware of which driver your laptop will need):
We are about (although it is up to you to understand and accept what I’m suggesting here, actually within my post #17) to use automated process (much easier one), brand new approach from Debian team:
Your sources.list is messed up. Debian does not carry byzantium suite, so this is an invalid entry:
deb http://deb.debian.org/debian bullseye non-free
And it causes all the problems, notably 404 not found. No wonder, Debian does not have byzantium, only PureOS does.
Next, if you mix Debian bullseye with PureOS byzantium, you will run into problems related same packages in the two distribution being slightly different versions. This might not bite you, if you only install a single package from Debian, but then again, it depends on a particular package and its dependencies.
Last, doing apt-get upgrade will install the highest version of every package, picked indiscriminately from both Debian and PureOS, and that is sure to either give problems, or it will effectively discard PureOS and switch you over to Debian. (PureOS lags behind Debian a little bit, and thus uses older versions of packages.)
isenkram-cli is part of main and therefore your proposal to use: apt-get install isenkram-cli might work well only if main, Debian 11 related one, added within sources.list, temporarily of course (certainly without executing apt-get upgrade, as you already noted, as not allowed under PureOS).
As there is no related feedback to your post, I rewrote your kind proposal up to my preferable liking (not because of I’m awaiting any relevant feedback either or thinking that what you already posted or I’m (re)posting here is everything needed, but might indeed do, at least in general under PureOS):
get root by sudo su
add to the /etc/apt/sources.list : deb https://ftp.debian.org/debian/ bullseye main non-free
execute apt-get update
execute apt-get install debian-archive-keyring
execute apt-get update
execute apt-get install firmware-linux-free
execute apt-get install firmware-misc-nonfree
execute apt-get install isenkram-cli
remove bullseye entry from /etc/apt/sources.list or mark it to something like this: ##deb https:// …, save/exit.