If you can't get any graphical output whatsoever, it is probably easiest to temporarily install a gpu, do the proxmox install and remove the gpu. Or you could remove the disk from the machine, install proxmox on the disk via a different computer (installing the disk in a different computer, installing proxmox, removing the disk and reinstalling it in your machine).
The other solution is doing a headless install: install debian headlessly (there are plenty of guides online if you are struggling) and then install proxmox on that base debian system. Here some resources on that. Make sure to install the current debian version (debian 11 bullseye as of writing).
unattended proxmox install (quite old wiki entry)
installing proxmox on debian 11
fully automatic installation of debian