Setup own ssl certificate

I just installed mattermost and want to enable traffic over TLS using my own SSL certificate. I moved my private key and full chain certificate files (private.key and fullchain.crt) in this path: /opt/mattermost/ssl and edited config.json like this:
“ListenAddress”: “:443”,
“ConnectionSecurity”: “TLS”,
“TLSCertFile”: “/opt/mattermost/ssl/fullchain.crt”,
“TLSKeyFile”: “/opt/mattermost/ssl/private.key”,
but after restarting service, I get error and service status is down. what did I wrong?

What does systemctl status mattermost.service say? My first guess is permissions issues on your cert and key files.