How to fix error "ModuleNotFoundError: No module named ..."?

I'm designing a website. The project structure is

project\ back_end\  __init.py__  main.py

In init.py,

from flask import Flaskdef create_app():    app = Flask(__name__)        return app

In main.py

from back_end import create_appapp = create_app()@app.route("/")def home():    return "hello world"

When I hit run in VS Code, there is error ModuleNotFoundError: No module named 'back_end'.

I type python -m back_end.main in the command line, the code works. I also tried to rename the back_end package to b. The error still persists. Is it related to defining an environment variable? I am using Ubuntu 24.04.