If the 64-bit virtual machines, such as VM alphaMELTS, are not showing up in VirtualBox (but 32-bit virtual machine are) then try the following. Thanks to Nikolaus Lechner for this solution:
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About the 64-bit Problem:
You have to enable the virtualizing tool (called sometimes VT-x) in the BIOS to be able to virtualize on a 64-bit System.
- Enter BIOS (F2, F10, F1, delete etc... depending on the computer)
- Though every BIOS is a little bit different you find the VT-x setting in "Advanced Settings", "system configuration" or "CPU settings".
- Enable every Virtualizing related option. sometimes there are 2 or they are called slightly different. but most of the time the term "Vt-X or "Virtualizing" is included.
- Save and Exit
- Shut down and reboot computer (no restart)
It should work now.
Also, you may need to disable Hyper-V. See here, for more details:
https://www.howtogeek.com/213795/how-to-enable-intel-vt-x-in-your-computers-bios-or-uefi-firmware/
If VirtualBox is still not working properly, try also disabling Fast Startup:
https://www.howtogeek.com/243901/the-pros-and-cons-of-windows-10s-fast-startup-mode/
Finally, note that the Windows Subsystem for Linux 2 (WSL2) requires Hyper-V to be enabled, so may not be compatible with VirtualBox.