Mac OSX Monterey (12.x) currently uses ports 5000 and 7000 for its Control centre hence the issue.
Try running your app from port other than 5000 and 7000
use this:
if __name__ == "__main__":
app.run(port=8000, debug=True)
and then run your flask file, eg: app.py
python app.py
You can also run using the flask command line interface using this command provided you have set the environment variable necessary for flask CLI.
flask run --port 8000
You can also turn off AirPlay Receiver in the Sharing via System Preference.
Related discussion here: https://developer.apple.com/forums/thread/682332
Update(November 2022):
Mac OSX Ventura(13.x) still has this problem and is fixed with the change in default port as mentioned above.