Really, if you are running this in anything production alike, just get a license!!
Well you cant get access to the enterprise repo's. so get a license instead.
but if you want to remove the annoying nag message:
sed -Ezi.bak "s/(Ext.Msg.show\(\{\s+title: gettext\('No valid sub)/void\(\{ \/\/\1/g" /usr/share/javascript/proxmox-widget-toolkit/proxmoxlib.js && systemctl restart pveproxy.service