Hi, @dakuhonoo
No worries. I know how it can be when you need to pick up the steam with a system that was originally set up by another personnel. Looking at the DataSource you share from the config.json file, it definitely looks different compared to the one in the documentation under Step #7:
"postgres://mmuser:<mmuser-password>@<host-name-or-IP>:5432/mattermost?sslmode=disable&connect_timeout=10" .
Since you have access to the server and database user credentials, can you make the necessary changes to the DataSource parameter and save the file before rebooting the server again and modify the channel again?