Your filesystem permissions look suspicious, not sure if this is really a problem, but we should fix them anyways. Please run the following commands to reset all folder permissions back to 775 (rwxrwxr-x) and all file permissions back to 664 (rw-rw-r–).
find /opt/mattermost/plugins -type d -print0 | xargs -0 chmod 775
find /opt/mattermost/plugins -type f -print0 | xargs -0 chmod 664
Once that’s done, please restart your Mattermost server and try again.