so firstly we need install pip
sudo apt install python-pip
if you need to upgrade your pip version so you can try this:-
pip install --upgrade pip
after install pip we need to install Flask
pip install flask
Postman is a powerful HTTP client for testing web services.
so friends visit on this link and Download it accordung to your OS:-
https://learning.getpostman.com/docs/postman/launching_postman/installation_and_updates/
https://www.getpostman.com/downloads/