That's a pretty clear error message. Netbox is trying to connect to a database on address 192.168.2.40 port 5432, and the connection is refused.
On the remote database server
192.168.2.40: either postgres isn't running, or it isn't configured to accept connections over the network (perhaps it's listening on 127.0.0.1), or it's blocking connections with firewall rules. It's an issue with configuration of your postgres server anyway.
However if 192.168.2.40 *is* your netbox server, and netbox and postgres are both running there, then you should set 'HOST': 'localhost' in your configuration.py