First download the module
wget https://pypi.python.org/packages/36/60/45f30390a38b1f92e0a8cf4de178cd7c2bc3f874c85430e40ccf99df8fe7/pysftp-0.2.9.tar.gz
tar xvzf pysftp-0.2.9.tar.gz
rm pysftp-0.2.9.tar.gz
sudo apt-get install libffi-dev
go to the folder pysftp-0.2.9 and run
sudo python setup.py install
Restart Odoo server
What is the error log? Or any more hints?