Flask: get error 404 when I try to access index

Solution
from app.models import Area, Sensor, Monitoring

from flask import Blueprint, request, jsonify
from flask.views import MethodView

bp = Blueprint('main', __name__)

@bp.route('/')
def hello_world():
    return 'Hello, World!'

# ...
def create_app():
    app = Flask(__name__)

    app.config['DEBUG'] = True
    app.config['SQLALCHEMY_DATABASE_URI'] = 'postgresql://%(user)s:%(pw)s@%(host)s:%(port)s/%(db)s' % POSTGRES
    app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = False

    db.init_app(app)

    from . import routes
    app.register_blueprint(routes.bp)

    return app
from flask import Flask
from flask_babel import Babel
from flask_marshmallow import Marshmallow
from flask_migrate import Migrate
from flask_security import Security, SQLAlchemyUserDatastore
from flask_sqlalchemy import SQLAlchemy


babel = Babel()
db = SQLAlchemy()
ma = Marshmallow()
migrate = Migrate()
security = Security()


_app_init_hooks = []
app_init_hook = _app_init_hooks.append


def create_app():
    app = Flask(__name__)

    for f in _app_init_hooks:
        f(app)

    return app


@app_init_hook
def _configure(app):
    """Load Flask configurations"""

    app.config.from_object(f"{__package__}.config")

    # optional local overrides
    app.config.from_pyfile("settings.cfg", silent=True)
    app.config.from_envvar("PROJECT_NAME_SETTINGS", silent=True)


@app_init_hook
def _init_extensions(app):
    """Initialise Flask extensions"""

    if app.env != "production":
        # Only load and enable when in debug mode
        from flask_debugtoolbar import DebugToolbarExtension

        DebugToolbarExtension(app)

    # Python-level i18n
    babel.init_app(app)

    # Database management (models, migrations, users)
    from .models import Role, User

    db.init_app(app)
    migrate.init_app(app, db)
    user_datastore = SQLAlchemyUserDatastore(db, User, Role)
    security.init_app(app, user_datastore)

    # Marshmallow integration (must run after db.init_app())
    ma.init_app(app)


@app_init_hook
def _setup_blueprints(app):
    """Import and initialise blueprints"""

    from . import users
    from .sections import BLUEPRINTS

    for blueprint in (*BLUEPRINTS, users.bp):
        app.register_blueprint(blueprint)

    return app
class CustomClient(Client):

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs):
        # Create a Flask context so we can access the SQLAlchemy session
        self._app = create_app()

    def add_reading(self, reading):
        with self._app.app_context():
            db.session.add(reading)
            db.session.commit()

    # ...