Nested virtualization is available on Windows 10 build 19636 and later. As far as I understand, AMD support will be officially available as part of Windows 11 and Windows Server 2022. Both products are expected in the second half of 2021.
Enable Nested Virtualization
Get the virtual machine intending to enable nested virtualization on using Powershell.
Now with the name of the VM, enable nested virtualization.
Set-VMProcessor -VMName 'Windows 11' -ExposeVirtualizationExtensions $True
Confirm Nested Virtualization is Enabled
To confirm nested virtualization is enabled for the VM, or any VM, use the following Powershell command.
(Get-VM 'proxmox ve 7.2' | Get-VMProcessor ).ExposeVirtualizationExtensions