Cannot connect PostgreSQL database with Flask app [duplicate]

I recently started learning the Flask framework, but now I am stuck, because - according to internet tutorials and documentation - my project is not working as it should be.

I have a larger project, but after seeing that problem I made a smaller one just to see why it is not working - but I still have no clue.

That is my 'smaller' code:

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'postgresql://postgres:test123@localhost/ranking'
db = SQLAlchemy(app)

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(80), unique=True)
    email = db.Column(db.String(120), unique=True)

    def __init__(self, username, email):
        self.username = username
        self.email = email

    def __repr__(self):
        return '<User %r' % self.username

@app.route('/')
def index():
    return "helo world"

if __name__ == '__main__':
    app.run()

and after running a python interpreter and typing there: from app import db I have no UserWarning (as in internet tutorials), but later - after typing: db.create_all() I received:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\Me\PycharmProjects\Flask\venv\lib\site-packages\flask_sqlalchemy\extension.py", line 868, in create_all
    self._call_for_binds(bind_key, "create_all")
  File "C:\Users\Me\PycharmProjects\Flask\venv\lib\site-packages\flask_sqlalchemy\extension.py", line 839, in _call_for_binds
    engine = self.engines[key]
  File "C:\Users\Me\PycharmProjects\Flask\venv\lib\site-packages\flask_sqlalchemy\extension.py", line 628, in engines
    app = current_app._get_current_object()  # type: ignore[attr-defined]
  File "C:\Users\Me\PycharmProjects\Flask\venv\lib\site-packages\werkzeug\local.py", line 513, in _get_current_object
    raise RuntimeError(unbound_message) from None
RuntimeError: Working outside of application context.

This typically means that you attempted to use functionality that needed
the current application. To solve this, set up an application context
with app.app_context(). See the documentation for more information.

What did I wrong? What should I do to make it work? Please help me.