Welcome to the Community,
the issue you are most likely facing is that WS2012 Essentials acts as a domain controller by default, and installing vCenter Server on a domain controller is not supported. What you may consider - unless you want to spend money for another Windows Server license - is to use the vCenter Server Appliance - which is a pre-configured Linux appliance - instead. With its embedded database, the appliance is currently supported for up to 5 hosts and up to 50 VMs.
André