If you follow the read.me instructions very carefully and also ensure that in Core Data Default Configuration, Used with CloudKit is check the app will compile and run in Xcode 13.1 with Simulator set to iPhone 12 iOS 15.
I had the same issue and fooled around with the Team, Bundle Identifier and iCloud Container names in Signing and Capabilities until they all matched. Xcode tries to help with the naming but its not intuitive and may mess up your names.
I've spent hours on this but for some reason Xcode doesn't import the data schema into the CloudKit Dashboard. All in all a very frustrating experience given the code in CoreDataStack.swift doesn't even look like any of the code that Nick Gillett presented at WWDC21. Nick presented code for a public database but the sample app file has configurations for private and shared that are completely different.
On top of all that they didn't post a SwiftUI version. It's UIKit only.
RichardinSydney
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Try "Practical Core Data" - Donny Wals. Ch7 on remote synchronisation uses JSON and decodable to import/sync into core data.