VT-x should be turned on
Virtualization requires that CPU's virtualization technology has been turned on in BIOS. In Intel x86 platform the virtualization technology is called VT-x. Check either directly from BIOS settings or use software like CPU-Z (Windows only).
Install Docker Toolbox
Note: Docker Toolbox also would like to install VirtualBox. If you already have VirtualBox installed don't reinstall it but skip VirtualBox installation step.