[Python] Why does running `flask run` on Windows result in "flask is not recognized as an...

I needed to setup Flask for windows, so I ran the following commands:

set FLASK_APP=application.py
set FLASK_DEBUG=1
set DATABASE_URL='postgres.......' =>(the credential given by the Heroku account)
flask run


But when I hit enter on the flask run, I get an error that says -

flask is not recognized as an internal or external command, operable program or batch file
​

Why is this not working? I've been trying to work on it but I'm still having the same error message.

What could be the issue?

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