I’m having the same issue. Xcode 13 beta 4 works however.

I'm having similar issues, here's my forum question. Trouble with WWDC21 sample code for CloudKit

I believe it might have something to do with NSSecureUnarchiveFromDataTransformerName, Core Data and CloudKit with NSPersistentCloudKitContainer. But I don't know why this would be in the unmodified demo from Apple? Any help much appreciated.

Actual Error: Context saving error: Error Domain=NSCocoaErrorDomain Code=134040 "Save operation could not be completed." UserInfo={NSAffectedStoresErrorKey=(     "<NSSQLCore: 0x102c2b760> (URL: file:///var/mobile/Containers/Data/Application/1505976C-354D-4B4F-A1DE-E6A312F01E99/Library/Application%20Support/private.sqlite)",         (         "<NSSQLCore: 0x102c2b760> (URL: file:///var/mobile/Containers/Data/Application/1505976C-354D-4B4F-A1DE-E6A312F01E99/Library/Application%20Support/private.sqlite)",         "<NSSQLCore: 0x102c301f0> (URL: file:///var/mobile/Containers/Data/Application/1505976C-354D-4B4F-A1DE-E6A312F01E99/Library/Application%20Support/shared.sqlite)"     ) ), NSUnderlyingException= threw while encoding a value.}