In that case you most likely need not to change anything in the BIOS of the VM - but rather:
* make sure that you have nested virtualization enabled on the PVE host: https://pve.proxmox.com/wiki/Nested_Virtualization
* select a CPU type for your VM which has the virtualization flag (the default kvm64 cpu does not) - if in doubt, and if it's a single machine the Host type CPU is likely a good choice
I hope this helps!
P.S. you can enter the "BIOS" of the VM by typing hitting the `Esc` key while it boots up (but there you can mostly select the boot-device)