Error : This kernel requires an x86-64 CPU,but only detected an i686 CPU.Unable to boot – please use a kernel appropriate for your CPU . This error is commonly seen when you try to install a new Operating System in Virtual Box.The error occur mostly comes due to user mistake.

Before going further ,first check the following thing if your Computer/laptop supports :

(Question 1) Do your Computer hardware has x86_64/64 Bit processor ?
(Question 2) Do your running Host Operating System is x86_64/64 bit ?
(Question 3) Do your computer supports virtualisation ?

If your all 3 above question’s answer is Yes. Then while installing new operating system in Virtual Box,you have not selected the option for 64 bit Operating system.

When you create a new virtual box template in starting ,you have to select 64 bit operating system.This is the only mistake,we generally do.See in given below screenshot,you have to select Version 64 bit respective of your Operating System you are installing.
eg. If you are installing Windows x86_64 bit then select Version Windows XP(64 bit) in Virtual Box