Hi,
I also ran to the issue after following the steps. can you help me to revert the setting and get rid of these errors before i proceed with your suggestions?
I ran the following command
sudo -u mattermost ./mattermost -assign_role -team_name="yourteam" -email="your@emailaddress.com" -role="system_admin" -config=/var/opt/gitlab/mattermost/config.json