Enable nested VT-X/AMD-V
I recently ran across this while attempting to install Android Studio inside a Linux guest VM. The option to enable nested VT-X/AMD-V in virtualbox settings (for the specific VM) was greyed out.
Shut the guest down, launch cmd.exe inside the VirtualBox installation folder
VBoxManage modifyvm guest-vn-name --nested-hw-virt on
The option tick box should be available and automatically selected.