I just thought to write this post as I recently came across with a requirement of running a virtual machine enabled with the nested virtualization capabilities on Ubuntu Linux Operating system which installed with Oracle VirtualBox software. I hope this is quite useful for the users who has the similar requirement and struggling to enable the nested virtualization for a Oracle VirtualBox Virtual Machine.
I created a Virtual Machine on the Oracle VirtualBox and “Enable Nested VT-x/AMD-V” option was grayed to when I was checking in the systems settings.

In order to enable this feature, I had to run the below command on the terminal.
VBoxManage modifyvm "<NAME_OF_THE_VM>" --nested-hw-virt on

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After that, the feature was successfully enabled in the system.

So, I was able to select the option and my Virtual Machine was ready to run the nested Virtual Machines inside its operating system.
I hope this small tweak will make your life more easier with Oracle VirtualBox.