Don't use this. It looks like, there is no community repository for PMG

DEB_VER="$(awk -F= '/^VERSION_CODENAME=/{print $2}' /etc/os-release)"
echo "deb https://enterprise.proxmox.com/debian/pmg ${DEB_VER} pmg-no-subscription" > /etc/apt/sources.list.d/pmg-no-subscription.list

mv /etc/apt/sources.list.d/pmg-enterprise.list{,.disabled}

sed -i.bak 's/NotFound/Active/g' /usr/share/perl5/PMG/API2/Subscription.pm
systemctl restart pmgproxy.service