Flask: Making my first web site

[snippsat]

Here is link to code.
So you need to have same folder structure and files as in Repo.
You can use DownGit and easy way to get a single folder in Repo if you not familiar with Git.
Just copy and paste link from Repo into DownGit.

So i did test download and place content in any folder and start python flaskblog.py

E:\all_flask\2019\02-Templates
λ python flaskblog.py
 * Serving Flask app "flaskblog" (lazy loading)
 * Environment: production
   WARNING: Do not use the development server in a production environment.
   Use a production WSGI server instead.
 * Debug mode: on
 * Restarting with stat
 * Debugger is active!
 * Debugger PIN: 334-187-997
 * Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
127.0.0.1 - - [23/Jun/2019 09:25:33] "GET / HTTP/1.1" 200 -
127.0.0.1 - - [23/Jun/2019 09:25:34] "GET /static/main.css HTTP/1.1" 200 -
127.0.0.1 - - [23/Jun/2019 09:25:43] "GET /about HTTP/1.1" 200 -
127.0.0.1 - - [23/Jun/2019 09:25:44] "GET / HTTP/1.1" 200 -
127.0.0.1 - - [23/Jun/2019 09:25:45] "GET / HTTP/1.1" 200 -

It work correct switch to about page and back home.

Error:
NameError: name 'About' is not defined

Also notice that about.html is not capitalize,as you have About.