Prepare a clean Debian system and upgrade to proxmoxve.

Debian11(bullseye) -> ProxmoxVE 7.x
  1. Configure hosts

Assuming your public network IP is 1.2.3.4 and your host name is sycblog, the /etc/hosts file is configured as follows:

127.0.0.1 localhost
1.2.3.4   sycblog.proxmox.com sycblog
# The following lines are desirable for IPv6 capable hosts
::1     localhost ip6-localhost ip6-loopback
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters

Verification:

hostname --ip-address
# return: 1.2.3.4
  1. Install PrxomoxVE
    echo "deb [arch=amd64] http://download.proxmox.com/debian/pve bullseye pve-no-subscription" > /etc/apt/sources.list.d/pve-install-repo.list
    wget https://enterprise.proxmox.com/debian/proxmox-release-bullseye.gpg -O /etc/apt/trusted.gpg.d/proxmox-release-bullseye.gpg 
    apt update && apt full-upgrade
    apt install proxmox-ve postfix open-iscsi
    systemctl reboot
  1. Test and complete the upgrade
https://1.2.3.4:8006
apt remove linux-image-amd64 'linux-image-5.10*'
update-grub
apt remove os-prober