Whats could be following the prerequisite for upgrading the vc from 5.0 to 6.0?
These KB links will provide a good information for your upgradation
Upgrading from vSphere 5.x to vSphere 6.0 Best Practices (2130664) | VMware KB
Upgrading to vCenter Server 6.0 best practices (2109772) | VMware KB
