InDesign doesn't allow InDesign files stored on Adobe's cloud system to utilize its powerful cross-reference feature. All the files must be kept on the local drive in order to utilize cross referencing. Adobe is encouraging us to store all our files in their cloud system (formerly CC), but doing so removes the possibility from cross-referencing information or even cross-referencing page numbers in a table of contents in Long Document multi file projects like books.

Adobe support says "I would like to inform you that you may not cross references with the other InDesign cloud document , how ever we are working on improving the app features and may be in future you may get that feature too"