Can't enable multiple CPU on VirtualBox

To increase performance you need to increase RAM and to use more than one CPU you need to enable "Virtualization technology" in the BIOS. Go to your BIOS options and search for "Virtualization technology" under "System settings" or similar. This is either called Intel-Vi-D or Intel-Vi with AMD processor. That name is then similar to AMD-V.

If you still have a problem with your VirtualBox not giving you an option to use multiple CPUs then you will most probably have to check if Hyper-V is installed in the "Turn Windows Features on or off". Which you might need to disable in order to use multiple CPU cores.

Some computers still might have no function towards virtualization use of multiple cores. That can be old computer models or more specific laptop computers. In some, you might need to check if there is a new BIOS version available for your computer.

For me I just had to forget the saved state with a right click on the virtual device.

Please make sure that you disable the Hyper-V. Go to Control Panel → Turn Windows features on or off → Uncheck Hyper-V → Restart your computer.