If you haven't have python 3 installed in your system, please install the Python. Also make sure you have pip installed.
Install the flask using pip
pip install flask
If you are on linux/mac, you have to use pip3 instead of pip
pip3 install flask
Run the main.py
python main.py
If you are on linux/mac, you have to use python3 instead of python
python3 main.py
Open the browser and go to 127.0.0.1:5000
, where you can see the index page