Binyamin pointed out this inconsistency Error in readme? for the times when you may need to run the application on a port other than port 5000.
In the file README.rst in the root from mflix.js.zip the lines 27-30 should read:
The API layer is fully implemented, as is the UI. The application is programmed
to run on port 5000 by default - if you need to run on a port other than
5000, you can edit the dotenv_win (if on Windows) or the dotenv_unix file
(if on Linux or Mac) in the root directory to modify the value of PORT.