As you are using KVM as your outer hypervisor, this is more of a KVM question as a Workstation one.
You can have nested x64 guests in a VMware workstation setup on the host that has a guest running VMware Workstation itself, that actually works quite well.
You can also run guests that have KVM as hypervisor, although admittedly I have not tested that with nested x64 guests (or at least can't remember that I did).
From your error it seems that KVM is capable of presenting a virtual VT-x implementation to its guest, but it does not support everything that is needed for VMware Workstation to be able to run x64 guests when running under KVM.
So the question is if you have control over the VT-x features as presented by KVM.
BTW, there's a special forum for nested virtualisation. (Nested Virtualization ) I'll move this thread over to there as you'll likely find better replies down there.
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