Unable to access system console, permission error in log

if that gives you permission denied running as root then you are probably missing something there...

Would also check your folders permissions and users with ls -la.

if that is a ubuntu I would go full root instead:

sudo -s
cd /opt/gitlab/embedded/bin
./mattermost roles system_admin traxwriter -c /var/opt/gitlab/mattermost/config.json

You also need to ensure that the mattermost folder and config.json folder are right.

Unhappily I don't know how gitlab does its thing so I can't advise further.