Hi there! When you get a chance, please introduce yourself here. I'd also encourage you to add a real picture as your avatar because it helps build community.
The error message suggests that there's something odd with your network config that is preventing Enketo from connecting to its local database. There are a lot of reasons why this could be, so you'll have to do some of your own troubleshooting.
Maybe you have a firewall up? Or are running other services on the server? Maybe it's Ubuntu 16 (we test with and recommend Ubuntu 18)?