Apart fom the above information, there are more considerations as follows. In order to run a multi-core virtual machine, VirtualBox needs a set of specific chip extensions usually enabled by default in recent computers and laptops. However, some vendors disable these extensions in the BIOS configuration, so you will not be able to run a Virtual Machine with two or more cores.
WARNING: CHANGING THE CONFIGURATION OF THE BIOS COULD BE DANGEROUS!!!!
DO NOT MODIFY IT UNLESS YOU KNOW WHAT YOU ARE DOING!!
If you want to solve it you will have to access the BIOS settings of your computer. Our recommendation is to read the manuals of your computer and check if there is a clear solution to enable this feature in your computer as every computer has a different set up for the BIOS settings. However, remember that this operation could be dangerous for your system if you modify something different than the virtualization extensions. If you do not feel comfortable modifying the BIOS settings, let the VM run with one core, as the wrapper should detect the problem and run with just one core.