before run your app.py you need to activate your server by this command
. venv/bin/activate
It looks like that you are using linux, if you are using ubuntutry: (similar for other linux distributions)
sudo apt-get install python-flask
It helped me install many packages that pip fails to install.
The app package and run.py file need to be in the same directory level.
app
run.py
app\ templates\ __init__.pyvenv\run.py
Above folder structure should fix.
P.S. pip install all necessary packages into virtual enviroment (venv in my example)
pip install
venv