/Users/rachelchoi/Desktop/Sparta/projects/prac/venv/bin/python /Users/rachelchoi/Desktop/Sparta/projects/prac/app.py 

 * Serving Flask app 'app'

 * Debug mode: on

Address already in use

Port 5000 is in use by another program. Either identify and stop that program, or start the server with a different port.

On macOS, try disabling the 'AirPlay Receiver' service from System Preferences -> Sharing.

localhost:5000 이라고 치면 사이트도 없고 계속 실행하면 이런 문구가 뜨네요.