I had a similar problem with flasgger.
The reason for that was that I always use
sudo pip install flask
but for some reason that's not always the way to go. Sometimes, you have to do just
pip install flask
Another gotcha is that sometimes people type pip install Flask
with the cap F
Posting this here in case somebody gets stuck. Let me know if it helped.
Useful Link: What is the difference between pip install and sudo pip install?