Installation

Followed this guide and this one.
Download Proxmox ISO.
Check the hash if you’re paranoid: 
$ sha256sum proxmox*.iso
Plug in USB drive. Don’t mount it. I had to format the drive for some reason. I hadn’t started taking notes.
$ sudo fdisk -l
$ mkfs.exfat /dev/sdc
Write ISO to the drive.
$ sudo dd if=/home/damo/Downloads/proxmox-ve_9.2-1.iso of=/dev/sdc bs=1M status=progress
Plug the drive into what will be the Proxmox machine and boot from the drive. Start the GUI installer and follow the instructions. Follow the guides. Save important info (password, static IP address). Access Proxmox GUI through the browser: [static IP]:8006

Problems

apt update && apt upgrade Errors

Err:4 https://enterprise.proxmox.com/debian/ceph-squid trixie InRelease     
    401  Unauthorized [IP: X.X.X.X 443]
Err:5 https://enterprise.proxmox.com/debian/pve trixie InRelease
    401  Unauthorized [IP: X.X.X.X 443]
Error: Failed to fetch https://enterprise.proxmox.com/debian/ceph-squid/dists/trixie/InRelease  401  Unauthorized [IP: X.X.X.X 443]
Error: The repository 'https://enterprise.proxmox.com/debian/ceph-squid trixie InRelease' is not signed.
Notice: Updating from such a repository can't be done securely, and is therefore disabled by default.
Notice: See apt-secure(8) manpage for repository creation and user configuration details.
Error: Failed to fetch https://enterprise.proxmox.com/debian/pve/dists/trixie/InRelease  401  Unauthorized [IP: X.X.X.X 443]
Error: The repository 'https://enterprise.proxmox.com/debian/pve trixie InRelease' is not signed.
Notice: Updating from such a repository can't be done securely, and is therefore disabled by default.
Notice: See apt-secure(8) manpage for repository creation and user configuration details.

Solution

Proxmox repos include enterprise repos by default. These can be swapped out for free ones.
https://forum.proxmox.com/threads/pct-command-not-found.91626/post-399490
https://pve.proxmox.com/pve-docs/pve-admin-guide.html#sysadmin_package_repositories

qm and pct “command not found”

https://forum.proxmox.com/threads/cli-commands-not-found.184733/

Solution

Add /usr/sbin to PATH

Can't ssh to local ip with VPN connected

Need to turn off VPN to connect to optiplex either from router wifi or extender wifi

Solution

Whitelist the subnet 
nordvpn whitelist add subnet 192.168.1.0/24
Source