iOS Xcode 12.0 Swift 5 'AppDelegate' is annotated with @main and must provide a main static function of type () -> Void or () throws -> Void

Change @main to @UIApplicationMain and ignore any resulting warnings.

Source: https://developer.apple.com/documentation/xcode-release-notes/xcode-12-release-notes

A newly-created iOS project using the Swift language may no longer build after enabling Mac Catalyst. (67885114)

Workaround: Replace the @main annotation on the App Delegate with @UIApplicationMain.

The Xcode 12.1 (or less) does not include the macOS 11 SDK, which the new SwiftUI app cycle requires for Mac apps.

You will have to install the Xcode 12.2 beta (candidate release) and use that until macOS 11 ships.