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