Installing elasticsearch-HQ I have cannot import name ‘Flask’ error

Hello,
Under kubuntu 18 I try to install elasticsearch-HQ,as written here http://docs.elastichq.org/installation.html#installation

But I got error :

pip install -r requirements.txt ... Successfully built alembic Installing collected packages: jmespath, itsdangerous, MarkupSafe, Jinja2, Werkzeug, click, Flask, pytz, six, aniso8601, Flask-restful, Flask-Script, marshmallow, flask-marshmallow, SQLAlchemy, marshmallow-sqlalchemy, urllib3, idna, certifi, chardet, requests, dogpile.cache,...
Code (markup):

Installing elasticsearch-HQ I have cannot import name 'Flask' error