Xcode 11: Canvas does not show up

You need to be on Catalina macOS version (10.15), as stated in official tutorial

Be warned: Catalina doesn't support 32-bit applications, some old apps will stop working after update.

you can still see the live view (without Catalina installed ) with the playgrounds. using UIHostingController.

import UIKit
import SwiftUI
import PlaygroundSupport



struct ContentView : View {
    var body: some View {

        Text("Hello World")
            .foregroundColor(Color.blue)
    }
}

// Present the view controller in the Live View window
PlaygroundPage.current.liveView = UIHostingController.init(rootView: ContentView())

To further add to the other answer, as per the official tutorial from Apple:

To preview and interact with views from the canvas in Xcode, ensure your Mac is running macOS 10.15 beta.

Unfortunate that I can't run it right now since the beta was just released and I don't have a spare Mac!