As per the thread below, I'm submitting a bug report for the inability to host nested VMs.
In my case, I have a Proxmox VM (PVE-Witness) running on unRAID. It's the third node of my Proxmox cluster.
When I try to fire up a VM on PVE-Witness that was just running on PVE-1, I'm met with the error:
TASK ERROR: KVM virtualisation configured, but not available. Either disable in VM configuration or enable in BIOS.
As requested, I attempted a similar task on a newly created Ubuntu 18.4 VM (Ubuntu). When creating the VM in Ubuntu, I'm met with: "Warning: KVM is not available."