Stuck on 56 build 52 on Centos 6.9 / No Updates

I am running (and stuck at) Cpanel 56.0 (build 52) with CENTOS 6.9 i686 virtuozzo in proxmox 3.3 CT for a long time. Everything works fine, except it no longer upgrades anything over version 56. There is a message says: "The last attempt to update cPanel & WHM was blocked." When I look at the details, it shows: Reasons for blocked updates. Please correct these issues and rerun updates

fatal: Newer releases of cPanel & WHM are not compatible with your operating system. Upgrade your OS, or use the Update Preferences screen to select a Long-Term Support release for use with your OS. Please see our