Host Server : DELL powerEdge R710
Processor : Six core
Entering into BIOS Manager> System Setup > Processor> Visualization = Enable
then Save & exit
"A common misconception about 64bit hardware: Having 64bit hardware/cpu does NOT mean you have VT-X/AMD-V as well, in other words: yes you can run a 64bit OS on 64bit hardware as a HOST, but a 64bit GUEST NEEDS VT-X/AMD-V to be active and enabled."
VirtualBox needs "VT-x" support when virtualizing multicore.