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Are you running PVE 6 or PVE7? For PVE7 you need to Bullseye PVE repo and not the Buster one.
 
From the Wiki:

Proxmox VE No-Subscription Repository​

This is the recommended repository for testing and non-production use. Its packages are not as heavily tested and validated. You don’t need a subscription key to access the pve-no-subscription repository.
We recommend to configure this repository in /etc/apt/sources.list.
File /etc/apt/sources.list
deb http://ftp.debian.org/debian bullseye main contrib
deb http://ftp.debian.org/debian bullseye-updates main contrib

# PVE pve-no-subscription repository provided by proxmox.com,
# NOT recommended for production use
deb http://download.proxmox.com/debian/pve bullseye pve-no-subscription

# security updates
deb http://security.debian.org/debian-security bullseye-security main contrib