I really like QubesOS, but you cannot run VMs inside a qube, or other things that require VMs like Docker Desktop for Linux, because the xen hypervisor does not support nested virtualization.
You
can. It's just neither recommended nor enabled by default.
https://forum.qubes-os.org/t/nested-virtualization/14790
Poke around /etc/libvirt/libxl and your particular VM's config file. You'll find some lines like:
<feature name='vmx' policy='disable'/>
<feature name='svm' policy='disable'/>
Enable it, and you should have working nested virtualization.
Shoot, as soon as I hit reply some neurons lit up and now I remember I was actually able to enable nested virtualization in QubesOS, and the relevant options in the VirtualBox preferences inside a qube became enabled once I did that, but whenever I tried booting any VM the whole system hanged. The same system and BIOS settings worked in Ubuntu to boot a nested VM in VirtualBox, so I think I had the BIOS settings correct. Anyhow, it seemed like a dead-end, so I abandoned it.
I'll have to look at it more. I mostly use AMD systems these days, which don't support nested virt in Xen, as I understand it, but it looks like it should work on Intel.