It is not possible to set 'Guest Operating System' for virtual machines in PVA MN, the list of OSes can be empty or incomplete.
The issue is caused by incorrectly generated PVA Agent file
The issue may take place when PVA Agent installation was interrupted on post-installation step or installation was completed with errors.
Reinstall PVA Agent on affected PCS node;
/opt/pva/agent/plugins/VZPBasicFunctionality/VZPBasicFunctionality.vocwith one from the healthy server of the same PVA Agent version.