What matters most is what version of Mattermost you upgraded FROM, that is, what version of Gitlab Omnibus you were updating from?
It's possible that your gitlab omnibus package's database migrations for the Mattermost database failed.
If you have no important history (your system isnt really live and in production yet) you can get back online by deleting and re-creating the mattermost database, which is a postgres database, I believe.
In a real production environment that would be a "disaster" for most companies, though, so you may need to manually run some migration scripts to get your database schema migrated.
Warren