If you have @StateObject var items: ItemModel = .init() in your ContentView, you should pass this model to the other views, for example @ObservedObject var items: ItemModel in your ItemView. Also you should have in your Observable class, items.append(Item(id: id, category: [category], image: image)) not Piercing(...). Currently you create a new and different ItemModel every time you show ItemView

Also you should have "@Published var items : [Item] = []" note, NO s in "[Item]". Conversely, in your delete, use "items.remove(at: index)"

Use .sheet(item: ...) instead, or capture the variable, like .sheet(isPresented: self.$showSheet) { [selectedItem] in ...}

Use .sheet(item: ...) instead, or capture the variable, like .sheet(isPresented: $isEditPresented) { [cardIndex] in ...}. Note, no need for @State in your EditCardView

If you cannot import Observation or import SwiftData it's because you do not have the required environment/target settings, such as target iOS 17, macOS 14. As mentioned in the comments in your StackOverflow question.

ForEach(lessons) { lesson in ...} is not a "normal" for loop. It is a View that needs to have other Views in it, not procedural code like you have. Put this type of code (using a "normal" for loop`), for example in .onAppear{ lessons.forEach{ lesson in ...} } Note that NavigationView is deprecated, use NavigationStack instead.

Use a separate @Model class Contact instead, works well for me. Even though Contact is Codable in your code, SwiftData does not store it like a model class.