@glenn exactly
I had the same problem:
This is how I solved it:
There is a virtualenv that is created by default and you need to activate it and install django-widget-tweaks inside it.
First cd into the home directory in the bash shell: cd ~
then cd .virtualenvs/ (remember to add the ".")
then cd into the virtualenv your web application is using... it's usually the same name as the website name
then cd bin/
then type 'source activate' to activate the virtualenv
If you are using python3 for your web app, then install django-widget-tweaks with pip3 -->
pip3 install django-widget-tweaks
if you are using python2 then -->
pip install django-widget-tweaks
When you are done, go to the 'Web' section and reload your web app. It should run fine
Hope this helps