Please shutdown the VM first and than try to reduce the vCPU down from 8 to 2 and restart the VM. Be insure that you expand the CPU config widget because the Socket/Core have changed of the past years. It sounds that you try to HotAdd vCPU and this is of couse a license feature which not comes with the free ESXi Hypervisor.
The free ESX Hypervisor only have that max. 8 vCPU limit per VM and not some kind of total # of vCPUs.
Regards,
Joerg