Select does not return values Postgres-11.4

The problem is due to python3-psycopg2. The latest pgadmin4 version requires psycopg2-2.8. But if you're on Debian/Ubuntu stable, apt installed v2.7. So you need to update it with pip :

sudo pip3 install -U psycopg2

Then update the pgadmin4 config to add the local python path:

nano ~/.config/pgadmin/pgadmin4.conf 
# or with a right click on the system tray icon
# add /usr/local/lib/python3.6/dist-packages to the PythonPath.

Same thing with python3.7 (just change the lib path)

Hope this helps.