VirtualBox only allowing 32 bit OS

The VirtualBox Support article I have a 64bit host, but can't install 64bit guests lists the following conditions for 64-bit guests:

1. You need to enable VT-x/AMD-v in the host PC BIOS.
2. Microsoft's Hyper-v must be uninstalled (Run OptionalFeatures.exe to check)
3. Avoid system level debuggers, other VM platforms, and some resident anti-virus applications.
4. Uninstall DeviceGuard or CredentialGuard if installed.

Check again the above conditions and especially if DeviceGuard or CredentialGuard were preinstalled by Dell. You may also try to disable your anti-virus and reboot.

If all conditions are met, then instead of the Repair of VirtualBox, try to uninstall it, reboot, install, reboot. If necessary, repeat this while booting in Safe mode.

If this still does not work, then some installed product conflicts with VirtualBox, which you will need to find. Autoruns is very useful in examining, disabling and re-enabling startup programs.

One possible problem I found which fixed it for me: Windows Defender => Device Security => Core isolation => Memory integrity must be DISABLED. (Had enabled it at some point in the past for some reason...)