Horizon Connection Server requires certain versions of VMware virtualization software.
If you are using vSphere, you must use a supported version of vSphere ESX/ESXi hosts and vCenter Server.
For details about which versions of Horizon are compatible with which versions of vCenter Server and ESXi, see the VMware Product Interoperability Matrix at http://www.vmware.com/resources/compatibility/sim/interop_matrix.php.