Hi,
Sorry about the trouble. You have deployed the developer version of the app that is not ready to be deployed.
If you are not making any important customization the one you want to download is the user version http://links.esri.com/storymaps/map_series_template_zip
We explain that in the instructions, see Esri/map-series-storytelling-template-js · GitHub
This should be more straightforward, we are going to fix this in next release and when you download the developer version you will receive a clear error message.