I'd had the same issue for my TX1320 M3(with CP400i).
As a result, I could install Proxmox in my TX1320 with ver 8.1.2, but I could not get that version from the official website(proxmox.com) because maybe 8.1.x was already outdated.
But luckily I could get it from https://web.archive.org/web/20240915175940/https://enterprise.proxmox.com/iso/.
I hope this post will help someone struggling with the same issue as us.
As a result, I could install Proxmox in my TX1320 with ver 8.1.2, but I could not get that version from the official website(proxmox.com) because maybe 8.1.x was already outdated.
But luckily I could get it from https://web.archive.org/web/20240915175940/https://enterprise.proxmox.com/iso/.
I hope this post will help someone struggling with the same issue as us.