You get this error when you try to run a VirtualBox inside on a physical machine which has not activated the support of VT-x from the bios.
But how to solve this issue when you want to run a VirtualBox inside Oracle VM guest, as afaik, accessing the BIOS is not available ?
The solution comes from how you create your VM guest. On Oracle VM, when you create an VM machine, it can support 4 domaines type:
You can refer to the official documentation: http://docs.oracle.com/cd/E27300_01/E27309/html/vmusg-vm-create.html
and Oracle Support Document 757719.1 (Oracle VM: Comparison of Guest Virtualisation Modes; HVM, PVM and HVM with PV Drivers) can be found at: https://support.oracle.com/epmos/faces/DocumentDisplay?id=757719.1
to understand in depth the difference between those types. Most of the VM I have created used HVM (the default option).
The tricky parts when you create a PVM guest are:
Below my vm.cfg file
vif = ['mac=00:21:f6:00:00:04,bridge=c0a80200']
OVM_simple_name = 'vbox-labs'
disk = ['file:/OVS/Repositories/0004fb00000300003527d62008dfea1f/VirtualDisks/0004fb000012000086c63ca973b241e4.img,xvda,w']
bootargs = 'http://webserver_host:webserver_port/ovm/dvd/'
uuid = '0004fb00-0006-0000-cb43-af3fa3edcb18'
on_reboot = 'restart'
cpu_weight = 27500
OVM_os_type = 'Red Hat Enterprise Linux 5'
cpu_cap = 0
maxvcpus = 8
OVM_high_availability = False
memory = 20480
OVM_description = ''
on_poweroff = 'destroy'
on_crash = 'restart'
bootloader = '/usr/bin/xenpvboot'
guest_os_type = 'linux'
name = '0004fb0000060000cb43af3fa3edcb18'
vfb = ['type=vnc,vncunused=1,vnclisten=127.0.0.1,keymap=fr']
vcpus = 8
OVM_cpu_compat_group = ''
OVM_domain_type = 'xen_pvm'