Hard to respond without details; are you referring to the standard Story Deep Copy app in the app catalog or the community Portfolio Item Deep Copy app? What exactly are you experiencing when you say "does not allow me to select..."? Can you not copy any story or are there specific ones that do and don't allow copy?
I've used both apps without exception, although we don't allow the use of child stories in my subscription so the regular Copy action in any user story meets our needs without the story deep copy app.