To disable iCloud sync of core data when the app launches, set the cloudKitContainerOptions to nil for the persistent store.
In your AppDelegate.swift file :
lazy var persistentContainer: NSPersistentCloudKitContainer = {
let container = NSPersistentCloudKitContainer(name: "your core data container name")
guard let description = container.persistentStoreDescriptions.first else {
fatalError("###\(#function): Failed to retrieve a persistent store description.")