BUG #16537: could not connect to database although database is running

The following bug has been logged on the website:

Bug reference: 16537
Logged by: William Frezno
Email address: for(dot)ggame(dot)playing(at)gmail(dot)com
PostgreSQL version: 12.3
Operating system: Parrot GNU/Linux 4.10
Description:

here is /var/log/postgresql/postgresql-12-main.log
2020-07-13 18:07:02.049 UTC [1092] FATAL: could not load server certificate
file "/etc/ssl/certs/ssl-cert-snakeoil.pem": No such file or directory
2020-07-13 18:07:02.050 UTC [1092] LOG: database system is shut down
pg_ctl: could not start server
Examine the log output.
#systemctl status postgresql
● postgresql.service - PostgreSQL RDBMS
Loaded: loaded (/lib/systemd/system/postgresql.service; disabled;
vendor p>
Active: active (exited) since Mon 2020-07-13 22:07:02 +04; 1h 19min
ago
Process: 1095 ExecStart=/bin/true (code=exited, status=0/SUCCESS)
Main PID: 1095 (code=exited, status=0/SUCCESS)

July 13 22:07:02 computer systemd[1]: Starting PostgreSQL RDBMS...
July 13 22:07:02 computer systemd[1]: Finished PostgreSQL RDBMS.

#psql
psql: error: could not connect to server: could not connect to server: No
such file or directory
Is the server running locally and accepting
connections on Unix domain socket "/var/run/postgresql/.s.PGSQL.5432"?

And I see port 5432 enabled on /etc/postgresql/12/main/postgresql.conf
port = 5432 # (change requires restart)
max_connections = 100 # (change requires restart)

What's wrong with postgresql server?