502 Bad Gateway - Debian 9

From your log:

Oct 26 12:17:05 Zammad zammad-worker-1.service[29386]: PG::ConnectionBad: could not connect to server: No such file or directory

Is your database setup correct? Zammad could not start because it’s missing a database. Maybe your config
/opt/zammad/config/database.yml is not correct

Regards