According to Proxmox VE, it is necessary to use motherboards and processors with KVM Full Virtualization support; however, this old motherboard only has VT-X and VT-D is not supported.
The operating system recommends to use RAID mode at chipset SATA. Double check if this option is enable in the BIOS.
System Requirements / Proxmox VE
Mike, Thanks for the reply, as for the RAID mode, I have not tested that as this was not necessary on any of the other machines that I mentioned as working fine, but I'll check it out. As for the need for full KVM Support...my guess is that this would be required in a Production environment for the full feature set, but for getting the OS up and running and creating a basic VM...which I have done and wrote that I have lightly tested this out on various pieces of Legacy equipment...to include a Single Core Pentium M based Dell M70 laptop without any difficulty, so I'd venture to say that this is not causing the Boot fail since the only system that I have that MAY support KVM is the Dell Precision T3500, but the same USB stick and the SSD work like a charm no matter what machine or how old I've plugged it into, except the Intel Boards...which is where both of these devices were actually installed from.