For anyone visiting this forum in 2022, there is an updated guide for Debian Bullseye and Proxmox 7.x:
https://pve.proxmox.com/wiki/Install_Proxmox_VE_on_Debian_11_Bullseye
BTW, @tom, I'm also having issues with the GUI installer and wanted to add a +1 for a text based installer (TUI). IMO...