Proxmox dokumentacija ( https://pve.proxmox.com/wiki/Package_Repositories ) pravi:

Proxmox VE No-Subscription Repository
  This is the recommended repository for testing and non-production use. Its packages are not as heavily tested and validated.
  You don’t need a subscription key to access the pve-no-subscription repository. We recommend to configure this repository in /etc/apt/sources.list

Sepravi: Rabiš naštimat no-subscription repository.
To narediš tako, da v fajl /etc/apt/sources.list (kot superuser oziroma sudo) dodaš nekaj takega (treba je bit pozorna ali je Debian Proxmox instalacije še vedno Buster, oziroma katera verzija je!!):

# PVE pve-no-subscription repository provided by proxmox.com, NOT recommended for production use.
deb http://download.proxmox.com/debian/pve buster pve-no-subscription

Celoten fajl /etc/apt/sources.list mora torej na koncu zgledat nekako takole:
deb http://ftp.debian.org/debian buster main contrib
deb http://ftp.debian.org/debian buster-updates main contrib

# PVE pve-no-subscription repository provided by proxmox.com, NOT recommended for production use.
deb http://download.proxmox.com/debian/pve buster pve-no-subscription

# security updates
deb http://security.debian.org/debian-security buster/updates main contrib

* prej preveri ali imaš pravilno naštiman repository - glej višje.
kot superuser oziroma sudo v terminalu izvedeš:
apt update
apt full-upgrade
še reboot:
systemctl reboot