I had to do it for my atom d2550s because of the odd hybrid x86/x86-64 systems they are. I had to install what ended up being linux mint debian edition 5 because that was the best way to get an OS on the odd bootloader system for various reasons, then upgraded to 6 to get to the latest debian, then I installed proxmox and removed all the debian stuff.

What do I do with something as weak as a pair of D2550s? Don't you worry about that. I've found uses for both. :P

It's an unusual use case, but it's one reason you might need to install debian before proxmox.