If you don't have paid subscription you should switch to 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.

Either delete /etc/apt/sources.list.d/pve-enterprise.list file or comment all lines in this file with #.

# deb https://enterprise.proxmox.com/debian/pve bullseye pve-enterprise

Create new file /etc/apt/sources.list.d/pve-no-subscription.list with content:

# 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

Read about Proxmox package repositories here: https://pve.proxmox.com/wiki/Package_Repositories