It's an easy fix. Sometimes the directions for the upgrade don't account for the source location of the APT REPO for ProxMox. Check your /etc/apt files and see where your repo is configured. If you are going from Buster to Bullseye for example, make sure that you have this line somewhere and the error should go away...
deb http://download.proxmox.com/debian/pve bullseye pve-no-subscription