You don’t need a license key to use the Proxmox No-subscription repository. It is suitable for home labs users, testing purpose and non-production use.
To enable Proxmox No-subscription repository, edit /etc/apt/sources.list file:
# nano /etc/apt/sources.list
And add the following lines:
deb http://ftp.debian.org/debian bullseye main contrib
deb http://ftp.debian.org/debian bullseye-updates main contrib
# security updates
deb http://security.debian.org bullseye-security main contrib
# PVE pve-no-subscription repository provided by proxmox.com,
# NOT recommended for production use
deb http://download.proxmox.com/debian/pve bullseye pve-no-subscription
Finally, update the repository list:
$ sudo apt update
$ apt full-upgrade