First remove the enterprise enterprise.proxmox repository
# rm /etc/apt/sources.list.d/pve.enterprise
#apt update && apt -y full-upgrade
Add the no-subscription or pve-test repositori :
#nano /etc/apt/sources.list
deb http://download.proxmox.com/debian buster pve-no-subscription
Go back to the GUI and check you can get updates!