You can install Proxmox by using their ISO and put it on a usb media by using balenaEtcher or rufus.
Proxmox is the os, its called a hypervisor os. But you can also install in ontop of Debian 12, in that case Debian 12 needs to be preinstalled.
Even when I installed Proxmox I couldn’t access the web interface
PVE installed from the ISO image is Debian with PVE packaged installed, so right after fresh install it is accessible on port 8006 and no iptables.
If you install on top of Debian, it also should work right out of box except if it was clashing with something already on your system or your firewall rules blocking it.
One last thing, of course, is that PVE asks you for static IP configuration during install, make sure it is on a routable subnet, same VLAN, etc. Also that your existing DHCP server residing on the same has not given out the IP to another machine.