Go into the virtual envirnoment - PFE-TrieuseConsigne/RPi GitHub Wiki
First of all, make sure you are in the directory with the virtual environment is installed and then, type the following command. For the exemple here, you should be on the /var/www/FlaskApp/FlaskApp/
source venv/bin/activate
After that, you should see that your root changed for
(venv) pi@raspberrypi:
Once in the virtual environment, we want to install Flask like so
sudo pip install Flask
To get out of the virtual environment, type the following command
deactivate