I having exact same issue as @samypr100 ..
It says I have 0 teams.
gitlab@gitlab:~/mattermost/bin$ sudo -u mattermost -i bash
mattermost@gitlab:~$ cd /opt/gitlab/embedded/service/mattermost
mattermost@gitlab:/opt/gitlab/embedded/service/mattermost$ /opt/gitlab/embedded/bin/mattermost -config='/var/opt/gitlab/mattermost/config.json' -upgrade_db_30
Please see http://www.mattermost.org/upgrade-to-3-0/
**WARNING** This upgrade process will be irreversible.
Have you performed a database backup? (YES/NO): YES
We found 0 teams.
Please pick a primary team from the list above: