Every single license comes from VMware... there is no HPE, DellEMC or what ever license. Yes there are ESXi Custom Install CDs(ISOs).

Now your fault    If you buy a Essentials(Plus) KIT you get 2 products which means 2 licenses. 

1x Instance vCenter Server 7 Essentials
6x CPU vSphere Essentials

For me it looks like that you try to enter your vCenter lic into your ESXi... you need the other key!

Note: You buy always the recent version which is right now vSphere 7. If you run vSphere 6 you need to perform a downgrade of the license within the CustomerConnect portal.

Regards,
Joerg