How do I log into a brand new install?

I found New Docker install, unable to create first Mattermost System Admin user and attempted to use the recommended command:

mmctl --local user create --email your@email.com --username yourusername --password yourpassword --system-admin

However, when attempting to run that, I’m getting the following error:

Error: Unable to create user. Error: : <untranslated>, failed to get Group with name=XXX: dial tcp [::1]:5432: connect: connection refused