I refer to the "Archived Special Event - Data Science Webinar Series" https://www.wolfram.com/wolfram-u/special-event/data-science-webinar-series/. To my opinion the file "RollingSales.nb" in http://wac.36f4.edgecastcdn.net/0036F4/pub/wolfram-u/notebook/DataScienceSession_1.zip is damaged and is not executable.
Is it possible to get a file without any problems?
Meantime it was possible for me to repair the file with help of article http://support.wolfram.com/kb/12423. So, it's not longer an issue for me.