When you install Ubuntu Server inside VirtualBox as a virtual machine you might run into problems. The installation itself works fine, but as soon as you reboot you’ll see this message
The kernel requires the following features not present on the CPU 0:6
Unable to boot – please use a kernel appropriate for your CPU
and Ubuntu server won’t boot. This is caused by ubuntu’s server kernel which supports Physical Address Extension (short PAE) which increases the maximum physical memory size from 4 Gib to 64 GiB, so that you can use more than 4Gib RAM even when you don’t use 64-Bit operating system.
VirtualBox supports this technology, but you’ve got to activate this feature inside the settings of your virtual machine…
… just enable the option “Enable PAE/NX”.
After you set the option you should be able to start your Ubuntu server inside VirtualBox