For LAN you don't need any port-forwarding you should be able to press Connect to IP then type the internal IP of the PC that started the LAN game and connect.
you also need to port-forward port number 4254 for the dedicated server to work properly but that error means that the your IP is already in use by another server so make sure you are not trying to run the server on both PC's at the same time.
Also except for port forwarding some routers require also "port-triggering" to be set up in order to open the ports to other clients