Way to reproduce:
- Create empty project in Unreal 4.17
- Build “For Distribution” and sign with distribution certificate with XCode 9 Beta 4+
- Upload to the AppStore
Result: Rejected, mail from the AppStore “Missing Asset Catalog - Your app is missing the Assets.car file in ‘blah.app’.”
Forum thread: Missing Asset Catalog - Cannot distribute app - Mobile Development - Unreal Engine Forums