A customer found that even though they set the hypervisorlaunchtype to off in the boot configuration, and they unchecked Hyper-V in the Turn Windows features on or off dialog, they found that hypervisor was still enabled. As a result, Oracle VM VirtualBox Manager refuses to create a 64-bit guest.
Someone from the kernel team explained that the hypervisor might still be enabled in the UEFI, so that's one thing to check.
Another thing that will force the hypervisor on is Virtualization-Based Security, which goes by the acronym VBS. This is confusing because to most people VBS stands for VBScript.