How Do You Troubleshoot the Error “ModuleNotFoundError: No module named ‘flask'”?

Problem scenario
You are running a Python program or running a command from the Python 3 interpreter. You get this error:

Traceback (most recent call last):
File "", line 1, in
ModuleNotFoundError: No module named 'flask'

What should you do to use Flask with Python 3?

Solution
Prerequisite
You need to have pip3 installed. You can do it with this if you are using Debian/Ubuntu Linux:

sudo apt-get -y install python3-pip

Procedures
Install Flask with pip3. Here is the command you should run: pip3 install flask