When I try to exec > python app.py , it throws this error

Traceback (most recent call last):
File "app.py", line 8, in
api = TwitterClient('@Sirajology')
File "/home/naman/Desktop/labeled-tweet-generator/twitter.py", line 14, in init
consumer_key = os.environ['TW9Jd0pnZll4NUrVGsKDu7hdc']
File "/usr/lib/python3.5/os.py", line 725, in getitem
raise KeyError(key) from None
KeyError: 'TW9Jd0pnZll4NUrVGsKDu7hdc'

Where did I do wrong? Thanks

Whenever app.py is run, it shows some error in line 3 of app.py stating that no module name flask is found. When I try to install flask via pip, it shows flask perfectly up!