Virtualbox enable nested vtx/amd-v greyed out

On my Ubuntu 18.04, I've installed VirtualBox 6.0 in order to have nested virtualization. Virtualization is enabled in my bios.

However, when I open the settings of my (powered off) virtual machine and go to System -> Processor, the option "Enable Nested VT-x/AMD-V" is greyed out and I cannot enable it.

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What is nested VTX?

Nested virtualization lets you run virtual machine (VM) instances inside of other VMs so you can create your own virtualization environments. To support nested virtualization, Compute Engine adds Intel VT-x instructions to VMs, so when you create a VM, the hypervisor that is already on that VM can run additional VMs.

Execute this:

$ VBoxManage modifyvm vm-name --nested-hw-virt on

For Windows

In Windows, go to VirtualBox installation folders -> type cmd on the bar (it will pop up cmd in that folder) -> type VBoxManage modifyvm YourVirtualBoxName --nested-hw-virt on -> enter.

Now it should been ticked.