The development server always shows this warning. The development server is not intended for use in production. It is not designed to be particularly efficient, stable, or secure. Use a production WSGI server instead. [See the deployment docs from Flask][1] for more information.
That warning is just a warning though, it's not an error preventing your app from running. If your app isn't working, there's something else wrong with your code.

[1]: https://flask.palletsprojects.com/en/2.2.x/deploying/

As of Flask 2.2, the development server always shows this warning. The development server is not intended for use in production. It is not designed to be particularly efficient, stable, or secure. Use a production WSGI server instead. See [the deployment docs from Flask](https://flask.palletsprojects.com/deploying/) for more information.
That warning is just a warning though, it's not an error preventing your app from running. If your app isn't working, there's something else wrong with your code.