Problem resolved. My error was I defined a "struct Notification" in the project which conflicts with Foundation "struct Notification". Thus the signature of the functions applicationDidFinishLaunching and applicationWillTerminate would not match!

Here is the code copy and pasted as is: p.s. it is linked with Boost lib (C\+\+) @main class AppDelegate: NSObject, NSApplicationDelegate, UNUserNotificationCenterDelegate {     var window: NSWindow! // Xcode warning here: Instance method 'applicationDidFinishLaunching' nearly matches optional requirement 'applicationDidFinishLaunching' of protocol 'NSApplicationDelegate' //     func applicationDidFinishLaunching(\_ aNotification: Notification) {         // Create the SwiftUI view that provides the window contents.         print("FIX applicationDidFinishLaunching")         let contentView = ContentView()         // Create the window and set the content view.         window = NSWindow(             contentRect: NSRect(x: 0, y: 0, width: 480, height: 300),             styleMask: [.titled, .closable, .miniaturizable, .resizable, .fullSizeContentView],             backing: .buffered, defer: false)         window.isReleasedWhenClosed = false         window.center()         window.setFrameAutosaveName("Main Window")         window.contentView = NSHostingView(rootView: contentView)         window.makeKeyAndOrderFront(nil)     } // Xcode warning here: Instance method 'applicationWillTerminate' nearly matches optional requirement 'applicationDidFinishLaunching' of protocol 'NSApplicationDelegate' //     func applicationWillTerminate(\_ aNotification: Notification) {         // Insert code here to tear down your application     } }