2. What to do if the Server does not Start

In case the python -m flask fails or flask does not exist, there are multiple reasons this might be the case. First of all you need to look at the error message.

2.1. Old Version of Flask

Versions of Flask older than 0.11 use to have different ways to start the application. In short, the flask command did not exist, and neither did python -m flask. In that case you have two options: either upgrade to newer Flask versions or have a look at the Development Server docs to see the alternative method for running a server.

2.2. Invalid Import Name

The -a argument to flask is the name of the module to import. In case that module is incorrectly named you will get an import error upon start (or if debug is enabled when you navigate to the application). It will tell you what it tried to import and why it failed.

The most common reason is a typo or because you did not actually create an app object.