[SOLVED] proxmox-backup-server package not available > PEBCAK

What is the actual full output (provide it in code-tags) of?:
  1. apt update
  2. apt install proxmox-backup-server
 
What is the actual full output (provide it in code-tags) of?:
  1. apt update
  2. apt install proxmox-backup-server
In addition to the output requested by @Neobin, also include the output of cat /etc/apt/sources.list /etc/apt/sources.list.d/* please.