Virtual box – this kernel requires an x86-64 CPU, but only detected an i686 CPU. Unable to boot – please use a kernel appropriate for your CPU

I had this when using remastersys which created an iso of my entire ubuntu os on a 64bit system.  When you create a virtual machine there is a ubuntu (64bit) option on first screen.  Use that and you wont get this error.

I think by default then virtual box machine is a 32bit so you have to change this or its trying to install a 64 bit operating systems on a 32 bit machine.