VT-x error when setting up VirtualBox on Ubuntu

What solved my issue was using less than 3 GB of ram in the virtual box session. I was originally attempting to utilize roughly 6 GB.

You are trying to allocate >3GB of RAM to the VM. This requires: (a) a 64 bit host system; and (b) true hardware pass-through ie VT-x.

In my case Memory(RAM) was not a problem, just VT-x wasn't enabled in BIOS. Or in some cases, CPU may not have support of VT at all.

To check whether your CPU supports it or not. try:

sudo apt-get install cpu-checker
sudo kvm-ok 

Second command will output if VT is enabled or not or if it's available at all.

If it does, go to BIOS and enable it. It should be named Intel Virtualization Technology, Intel VT-x, Virtualization Extensions, Vanderpool, or something similar.