I am getting this error when moving a vm from one old standalone 5.1 esxi host to vcenter 6.7
any idea how I can override this?
It looks like ESXi 5.1 didn't properly check the MAC address, that has been configured. It should actually have refused it.
Anyway, if you need to preserve that MAC address, you may edit the VM's options, and add "ethernet0.CheckMACAddress" set to "FALSE" to the Configuration Parameters..
André