This error shows up on windows when one tries to use pip in the command prompt. To solve this error on windows, you must declare path variable by following these steps:
Step 1 - Right click on My Computer or This PC
Step 2 - Click on Properties
Step 3 - Click on Advanced System Settings
You will find a section called system variables.
Click on Path from the list of variable and values that shows up there.
After clicking on path click edit.
You will find a New button in the pop up.
Click that and paste the location of the python35 or python36 folder (The location you specified while installing python) followed by “\Scripts” there.
For me its “C:\Users\a610580\AppData\Local\Programs\Python\Python35-32”
so I type “C:\Users\a610580\AppData\Local\Programs\Python\Python35-32\Scripts”
Click Ok to close all windows and restart your command prompt.
I repeat - restart your command prompt.