The URL is not a URL on your MAIN host it's a whole different virtual host with a different DNS name.
If you read the docs twice, you might find that the first time you missed some essentials, and the second time you grabbed on to them. That's what happened to me...
- If your machine has a DNS entry for casgitlab.casalelug.ch, go add a second DNS entry for chat.casalelug.ch, or whatever you want your mattermost to have.
- Follow instructions to set it up. Including creating API key.
- Because mattermost is a multitenant application you will find that visiting http://chat.casalelug.ch" will request that you now create a Team (until there's a team, there's no other feature in Mattermost that you can access).
- After you have a team you will see the main mattermost ui.