Adding a bibliography on a Mac can be very slow for very long documents. It should eventually work. You might need to run it overnight. (An alternative for very large documents is to use a PC when doing this final step. It's faster on Windows, because Word allows Zotero to interact with it faster.)

If you need more specific help, post more specific information (exactly which version of Zotero and Word, maybe a Debug ID if it crashes).

Similar discussions:
https://forums.zotero.org/discussion/comment/326898
https://forums.zotero.org/discussion/76859/word-365-for-mac-citations-very-slow-bibliography-crashing/p1
https://forums.zotero.org/discussion/comment/333284
etc.