What a pain! Same problem trying to see 64bit guests in VirtualBox. I have a Ryzen-5 2400G processor with Gigabyte UEFI model B450MDS3H ver F42 and BIOS ID: 8A16BG05. After much screwing around, here is where to find the setting: After you are in the Gigabyte UEFI main page, click on M.I.T. The select "Advanced Frequency Settings," then under that select "Advanced CPU Settings" Finally, enable SVM (secure virtual machine.) Reboot. Voila!