PostgreSQL : FATAL: could not open log file "/path/to/log/filesystem/postgresql-Sun.log": No such file or directory
User, which runs postgresql, often postgres, must be able to traverse to this /path/to/log/filesystem/ (often /var/log/postgresql), specified by log_directory config option and be able to write files there. If this path does not exist, you will get No such file or directory error.
You can try switching to this user (e.g. su postgres) and check cd /path/to/log/filesystem (whatever this path really is) and if that throws error, then create the directories (e.g. mkdir -p /path/to/your/log_directory).
There's also a possibility that service manager changes mount points for the service, like systemd is capable of.