Running the reasonably up-to-date GitLab v8.5.3 with Mattermost v2.0.0 Build 5950.
I had the System Console appearing in my menu...then it disappeared. Only thing I can think of that may have caused it to disappear was I changed my email address in my GitLab account that the Mattermost account was twinned to through SSO. I changed my GitLab email address back to what it was originally but this did not fix the problem.
So I followed the suggestion above to execute the "./mattermost -assign_role -team_name="TeamName" -email="myaddress.com" -role="system_admin"" command to re-assign the admin role to the team but it fails with the error message "panic: runtime error: invalid memory address or nil pointer dereference".
Based on another error report I then tried with the "sudo -u" prefix and the "-config="/var/opt/gitlab/mattermost/config.json" option but got the "panic: Failed to load system translations for 'en'" error.
So, based on yet another error report I copied the en.json file over to the /opt/gitlab/embedded/bin/ directory and now it fails with the "panic: invalid character '<' looking for beginning of value" error.
Sigh...there has to be an easier way to get logged in as admin in Mattermost!
What do I try next...any suggestions?