Hello there!
I solved this by editing the users table in the mattermost_production database; A query like this:
UPDATE users SET roles = "admin system_admin" where username = "your_username";
To access the mattermost database in the omnibus installation:sudo -u gitlab-psql /opt/gitlab/embedded/bin/psql -h /var/opt/gitlab/postgresql -d mattermost_production
Cheers!