Step 1.) Upgrade to Debian 11 first
The process to go to Debian 12 is not as smooth as 11, when trying to upgrade from Debian 10. In fact, it doesn't work directly, so you'll first need to follow this guide to update to Debian 11, reboot and come back here if successful.
Step 2.) Update sources.list
Update your /etc/apt/sources.list like this:
deb http://........
This happens during an apt update and is related to an issue with sources.list, which is particularly troubling, if you are doing a "live-build".
P: Configuring file /etc/apt/sources.list
Hit:1 http://deb.debian.org/debian bullseye InRelease
Get:2 http://deb.debian.org/debian bullseye-updates InRelease [39.4 kB........
Are you getting this error in Proxmox while trying to apt update or install Ceph?
apt update
Hit:1 http://security.debian.org bullseye-security InRelease
Err:2 https://enterprise.proxmox.com/debian/pve bullseye InRelease
401 Unauthorized [IP: 144.217.225.162 443]
Hit:3 http://ftp.hk.debian.org/debian bullseye InRelease ........
I decided on using yum to help me decide even though I normaly use proftpd I decided to see what else I could find.
yum search ftp
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
* rpmforge: ftp-stud.fht-esslingen.de
* base: mirrors.netdna.com
* updates: updates.interworx.info
* addons: yum.singlehop.com
* extras: mirrors.netdna.com
rpmforge........