Installed PostgreSQL-11 in Ubuntu 18.04.02 Server Edition: No existing local cluster is suitable as a default target

I installed PostgreSQL-11 in Ubuntu 18.04.02 Server Edition using package
manager: sudo apt-get install postgresql-11. But now I'm not able to call
psql:

(base) marco(at)pc:~$ sudo su -l postgres
[sudo] password for marco:
No directory, logging in with HOME=/
postgres(at)pc:/$ psql
Warning: No existing local cluster is suitable as a default target.
Please
see man pg_wrapper(1) how to specify one.
Error: You must install at least one postgresql-client-<version> package
postgres(at)pc:/$

I tried to specify the port to use but it asks for a postgresql-client:

(base) marco(at)pc:~$ sudo -u postgres /usr/bin/psql -p 5432
Error: You must install at least one postgresql-client-<version> package

But the latest postgresql-client is already installed:

(base) marco(at)pc:~$ sudo apt-get install postgresql-client-11
Reading package lists... Done
Building dependency tree
Reading state information... Done
postgresql-client-11 is already the newest version (11.5-1.pgdg18.04+1).
0 to upgrade, 0 to newly install, 0 to remove and 0 not to upgrade.

How to solve the problem? Looking forward to your kind help.
Marco