We recently found out that the Assignment Submission Folders default to a "hidden from users" view.  This is a temporary problem and will be fixed in November's 2017 release.  This is how the Assignment Submission Folder will appear if it is "hidden from users" (look for the eyeball icon).

In the meantime, when creating a new Assignment Submission Folder, please select the "Restrictions" tab and uncheck the "Hidden from Users" option.