TypeError: missing dsn and no parameters when I change my database

I'm trying to change a simple chat application to a more complex one by following this tutorial . Now I am deciphering how to change my database. The teacher chose Flask-SQLAlchemy and SQLite but I've already started with PostgreSQL. So I try to modify my application little by little to be able to use it, which seems to be more serious.

However, I tried the simple command > from app.models import User and got a TypeError: missing dsn and no parameters .

(MyFlaskAppEnv) mike@mike-thinks:~/Programing/Rasa/myflaskapp$ python3.6
Python 3.6.5 (default, May  3 2018, 10:08:28) 
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from app.models import User
/home/mike/.local/lib/python3.6/site-packages/psycopg2/__init__.py:144: UserWarning: The psycopg2 wheel package will be renamed from release 2.8; in order to keep installing from binary please use "pip install psycopg2-binary" instead. For details see: <http://initd.org/psycopg/docs/install.html#binary-install-from-pypi>.
  """)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/mike/Programing/Rasa/myflaskapp/app.py", line 31, in <module>
    conn = psycopg2.connect(DATABASE_URL)
  File "/home/mike/.local/lib/python3.6/site-packages/psycopg2/__init__.py", line 127, in connect
    raise TypeError('missing dsn and no parameters')
TypeError: missing dsn and no parameters
Error in sys.excepthook:
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/apport_python_hook.py", line 63, in apport_excepthook
    from apport.fileutils import likely_packaged, get_recent_crashes
  File "/usr/lib/python3/dist-packages/apport/__init__.py", line 5, in <module>
    from apport.report import Report
  File "/usr/lib/python3/dist-packages/apport/report.py", line 30, in <module>
    import apport.fileutils
  File "/usr/lib/python3/dist-packages/apport/fileutils.py", line 23, in <module>
    from apport.packaging_impl import impl as packaging
  File "/usr/lib/python3/dist-packages/apport/packaging_impl.py", line 23, in <module>
    import apt
  File "/usr/lib/python3/dist-packages/apt/__init__.py", line 23, in <module>
    import apt_pkg
ModuleNotFoundError: No module named 'apt_pkg'

Original exception was:
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/mike/Programing/Rasa/myflaskapp/app.py", line 31, in <module>
    conn = psycopg2.connect(DATABASE_URL)
  File "/home/mike/.local/lib/python3.6/site-packages/psycopg2/__init__.py", line 127, in connect
    raise TypeError('missing dsn and no parameters')
TypeError: missing dsn and no parameters

Should I comment on my old database?

# Online Database
conn = psycopg2.connect(DATABASE_URL)