Adding more cores to a VMClient : Feature 'Hot-Pluggable virtual HW' is not licensed with this edition

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I am trying to add more cores to a VM client running on the following HP Server and I am getting the following error:

Feature ‘Hot-Pluggable virtual HW’ is not licensed with this edition.

What do I need to purchase in order to add more cores? The server has 1 CPU and 6 cores?

Product: VMware vSphere 6 Hypervisor Licensed for 1 physical CPUs (unlimited cores per CPU)
License Key: H129K-0UL9L-28Y88-XXXX-XXXX
Expires: Never

Product Features:
Up to 8-way virtual SMP

Iron Castle Systems