If an ESXi host is licensed with the free version of the license key, you cannot add it to vCenter Server. This license does not contain the VirtualCenter Agent, which is necessary to manage a host with vCenter Server. This feature remains locked as long as the host is licensed with the free version of the license key.
To add ESXi hosts to vCenter Server, you must license the ESXi hosts with:
Licensing an ESXi host with vSphere Essentials allows you to manage three ESXi hosts using vCenter Server. For more information on purchasing vSphere Essentials, see VMware vSphere and VMware vCenter Server Pricing.
vSphere Standard, Advanced, Enterprise, or Enterprise Plus
Licensing the ESXi host with one these products unlocks the add-on features that allow you to manage ESXi hosts using vCenter Server. For more information about these products, see Compare vSphere Editions.
For more information on licensing an ESX/ESXi 4.x host, see Licensing ESX 4.x, ESXi 4.x, and vCenter Server 4.x (1010839).
For more information on licensing an ESXi 5.x host, see Licensing ESXi 5.x and vCenter Server 5.x (2014295).
Based on VMware KB 1018275