Unable to install a VM into another VM

My host computer is running Windows 10, and my guest is Ubuntu in VirtualBox. I am trying to run a 3rd VM on the Ubuntu, but VirtualBox says:

Failed to open a session for the virtual machine Windows XP. VT-x is not available. (VERR_VMX_NO_VMX).

It also returned an error code of 0x80004005

I have virtualization enabled, of course. It's not even an option to turn off in my BIOS (MSI z170a). My machine is very high end, so I know it can run it, I've ruled out everything I can.

What you're trying to do is nested virtualization. Its not supported in virtualbox and in no way is a priority for them.

You'll need to run the XP VM in the host instead.

In THEORY you could run plain old QEMU in the VM (it emulates the whole processor and pretty much the whole system) to do this but the performance will be really really bad.