Doing a search for your error message leads me to believe that the cause of the problem is that Color doesn't conform to Codable. You have to write a transformer for Color that inherits from NSSecureUnarchiveFromDataTransformer. Inheriting from NSSecureUnarchiveFromDataTransformer will make the error/warning go away. Take a look at the following Stack Overflow questions: https://stackoverflow.com/questions/77831799/nskeyedunarchivefromdata-should-not-be-used-to-for-un-archiving-and-will-be-r https://stackoverflow.com/questions/62589985/nskeyedunarchivefromdata-should-not-be-used-to-for-un-archiving-and-will-be-re The second question has an answer that shows how to create a transformer.

What book are you referring to when you say "In the book"? What are you trying to do when you click the Quick Look button? Telling us what you are trying to do can help us tell you what you have to do in Xcode 26 to do the thing you are trying to do.

The Mac App Store might not accept command-line apps. They may require apps to be in a Mac app bundle. One way to build your app for the Mac App Store is to create a simple Mac GUI app project in Xcode and bundle your command-line app with the GUI app. Use the Process class to run the command-line app. You will have to conform to the App Sandbox guidelines to get approved on the Mac App Store. Many Mac apps that bundle command-line apps aren't in the Mac App Store because of the App Sandbox restrictions. You might have to distribute your app from your own website.