Execute the following to upgrade your system:
mv /etc/apt/sources.list.d/pve-enterprise.list /etc/apt/sources.list.d/pve-enterprise.bak
echo "deb http://download.proxmox.com/debian/pve buster pve-no-subscription" > /etc/apt/sources.list.d/pve-no-subscription.list
apt update
apt full-upgrade