If you have VT-x enabled on your BIOS, but this option is grayed out on Virtualbox interface, you can do the following to fix:
You may need to locate to the bin folder of virtualbox, if you’re on Windows, the following command should work.
VBoxManage modifyvm vm-name --nested-hw-virt on # On windows VBoxManage.exe modifyvm vm-name --nested-hw-virt on
Please change vm-name to your VirtualBox machine name