The requested URL was not found on the server. If you entered the URL manually please check your spelling and try again

Based on your clarifying comments, it appears your browser is accessing a previous version of your app server.

I have run into this problem myself when using the PyCharm IDE. On Windows, the way to shut down these old versions of the server is to press either Ctrl+Alt+Delete or Ctrl+Shift+Esc (Windows 10) to view your list of running processes. Then scroll down until you see processes named Python and shut them all down.

I don't know if it's due to some other python application running or something But when I was a beginner in flask I got this error because of writing

app.route('/')

instead of @app.route('/)

:|

Though without @ it ran the server.

See whether its the case