TASK ERROR: command 'apt-get update' failed: exit code 100

Please run the command (apt-get update) on the host console (or via SSH) and show the output. It probably contains a message that gives more information, like here: https://forum.proxmox.com/threads/command-apt-get-update-failed-exit-code-100.42387/
Code:
root@prox:~# apt-get update
Hit:1 http://ftp.it.debian.org/debian bookworm InRelease
Hit:2 http://security.debian.org bookworm-security InRelease   
Get:3 http://ftp.it.debian.org/debian bookworm-updates InRelease [55.4 kB]                 
Err:4 https://enterprise.proxmox.com/debian/ceph-quincy bookworm InRelease                 
  401  Unauthorized [IP: 51.91.38.34 443]
Err:5 https://enterprise.proxmox.com/debian/pve bookworm InRelease
  401  Unauthorized [IP: 51.91.38.34 443]
Reading package lists... Done
E: Failed to fetch https://enterprise.proxmox.com/debian/ceph-quincy/dists/bookworm/InRelease  401  Unauthorized [IP: 51.91.38.34 443]
E: The repository 'https://enterprise.proxmox.com/debian/ceph-quincy bookworm InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: Failed to fetch https://enterprise.proxmox.com/debian/pve/dists/bookworm/InRelease  401  Unauthorized [IP: 51.91.38.34 443]
E: The repository 'https://enterprise.proxmox.com/debian/pve bookworm InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
root@prox:~#
 
Code:
root@prox:~# apt-get update
Hit:1 http://ftp.it.debian.org/debian bookworm InRelease
Hit:2 http://security.debian.org bookworm-security InRelease  
Get:3 http://ftp.it.debian.org/debian bookworm-updates InRelease [55.4 kB]                
Err:4 https://enterprise.proxmox.com/debian/ceph-quincy bookworm InRelease                
  401  Unauthorized [IP: 51.91.38.34 443]
Err:5 https://enterprise.proxmox.com/debian/pve bookworm InRelease
  401  Unauthorized [IP: 51.91.38.34 443]
Reading package lists... Done
E: Failed to fetch https://enterprise.proxmox.com/debian/ceph-quincy/dists/bookworm/InRelease  401  Unauthorized [IP: 51.91.38.34 443]
E: The repository 'https://enterprise.proxmox.com/debian/ceph-quincy bookworm InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: Failed to fetch https://enterprise.proxmox.com/debian/pve/dists/bookworm/InRelease  401  Unauthorized [IP: 51.91.38.34 443]
E: The repository 'https://enterprise.proxmox.com/debian/pve bookworm InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
root@prox:~#
You need a support subscription to use the enterprise repositories. If you want to use the no-subscription repositories, you need to also disable the enterprise repositories. Lots of threads on this forum about this (like the one I linked before) and also documentation in the manual: https://pve.proxmox.com/pve-docs/pve-admin-guide.html#sysadmin_package_repositories
 
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Reactions: gianry
The error has changed to the enclosed must be a missing repository still somewhere
No you have one too many (to which you have no access without a subscription),
Err:5 https://enterprise.proxmox.com/debian/pve bookworm InRelease
401 Unauthorized [IP: 51.91.38.34 443]
If you want to use the no-subscription repositories, you need to also disable the enterprise repositories.
Look /etc/apt/sources.list.d/pve-enterprise.list and the other usual places for repositories. Nowadays, it's also in the Proxmox web GUI.