Problem with repository and upgrade

root@proxmox101b:/etc/apt# apt update
Hit:1 http://security.debian.org/debian-security bullseye-security InRelease
Hit:2 http://ftp.debian.org/debian bullseye InRelease
Get:3 http://download.proxmox.com/debian/pbs bullseye InRelease [3,067 B]
Hit:4 http://ftp.debian.org/debian bullseye-updates InRelease
Err:3 http://download.proxmox.com/debian/pbs bullseye InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY...