On Debian Bookworm

Follow Install Proxmox VE on Debian 12 Bookworm

What is missing:

• apt install dnsmasq

WARN: missing 'source /etc/network/interfaces.d/sdn' directive for SDN support!

It seems like you have automatic DHCP active, but no gateway configured in the subnet.

/etc/network/interfaces

source /etc/network/interfaces.d/*