Hi @ichi-no-eda,
Have you tried curling localhost to see if that works? For example curl http://localhost:8065. That should work if Mattermost is running, regardless of the state of NGINX.
If that doesn't return a Mattermost HTML page, then Mattermost probably isn't running, possibly because it failed on start-up. Inside the directory where you installed mattermost there should be a log file under mattermost/logs/mattermost.log. Open up that file and look for any errors. If you see any they might help you identify the issue. You can post them here as well if you'd like some more help 
Also if you this instance is only for testing, and the clients that are going to be connecting to your Mattermost server all have the ability to connect directly to it, then you can skip NGINX if you don't need SSL (HTTPS) connections. You can just point your browser at http://your.url.here:8065/ and it should work correctly
Let me know if that helps and if you have any more questions