All files including with the app install on device must be in the unalterable app bundle which makes the files readonly.
If you include a Core Data persistent store file with the app, it resides inside the app bundle and can only be added to the persistent store coordinator as as a readonly store. Any attempt to write to it will produce an error.
You usually do need a readwrite store so the solution is to use standard file operations to copy the included persistent store file from the app bundle and then into the app's Documents or Library folder. When the app launches, the app delegate checks for the presence of the store before initializing the Core Data stack and copies it over if it hasn't already done so.
You also have the more advanced option of using two stores for one stack, one readonly and another readwrite. However, that method introduce complexity into the data model and is usually not needed.