Error Domain=kAFAssistantErrorDomain Code=216 "(null)"

Basically I am learning ios speech recognition module following this tutorial:https://medium.com/ios-os-x-development/speech-recognition-with-swift-in-ios-10-50d5f4e59c48

But when I test it on my iphone6, I always got this error: Error Domain=kAFAssistantErrorDomain Code=216 "(null)"

I searched it on the internet, but find very rare info about this.

Here is my code:

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import UIKit
import AVFoundation
import Speech
class ViewController: UIViewController, SFSpeechRecognizerDelegate {
  
    /
    let audioEngine = AVAudioEngine()
    let speechRecognizer: SFSpeechRecognizer? = SFSpeechRecognizer(locale: Locale.init(identifier: "en-US"))
    var request = SFSpeechAudioBufferRecognitionRequest()
    var recognitionTask: SFSpeechRecognitionTask?
    var isRecording = false
  
    override func viewDidLoad() {
        /
        /
        self.requestSpeechAuthorization()
    }
    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        /
    }
  
    /
    @IBOutlet weak var detectText: UILabel!
    @IBOutlet weak var startButton: UIButton!
  
    /
    @IBAction func startButtonTapped(_ sender: UIButton) {
        if isRecording == true {
          
          
            audioEngine.stop()
/
/
/
            audioEngine.inputNode?.removeTap(onBus: 0)
            /
            self.request.endAudio()
          
            /
            if let recognitionTask = recognitionTask {
                recognitionTask.cancel()
                self.recognitionTask = nil
            }
          
          
            /
            /
            isRecording = false
            startButton.backgroundColor = UIColor.gray
        } else {
            self.recordAndRecognizeSpeech()
            isRecording = true
            startButton.backgroundColor = UIColor.red
        }
    }
  
    /
    func showAlert(title: String, message: String, handler: ((UIAlertAction) -> Swift.Void)? = nil) {
        DispatchQueue.main.async { [unowned self] in
            let alertController = UIAlertController(title: title, message: message, preferredStyle: .alert)
            alertController.addAction(UIAlertAction(title: "OK", style: .cancel, handler: handler))
            self.present(alertController, animated: true, completion: nil)
        }
    }
  
    /
    func recordAndRecognizeSpeech() {
        /
        guard let node = audioEngine.inputNode else { return }
        let recordingFormat = node.outputFormat(forBus: 0)
        node.installTap(onBus: 0, bufferSize: 1024, format: recordingFormat) { buffer, _ in
            self.request.append(buffer)
        }
        audioEngine.prepare()
        do {
            try audioEngine.start()
        } catch {
            self.showAlert(title: "SpeechNote", message: "There has been an audio engine error.", handler: nil)
            return print(error)
        }
        guard let myRecognizer = SFSpeechRecognizer() else {
            self.showAlert(title: "SpeechNote", message: "Speech recognition is not supported for your current locale.", handler: nil)
            return
        }
        if !myRecognizer.isAvailable {
            self.showAlert(title: "SpeechNote", message: "Speech recognition is not currently available. Check back at a later time.", handler: nil)
            /
            return
        }
        recognitionTask = speechRecognizer?.recognitionTask(with: request, resultHandler: { result, error in
            if let result = result {
              
                let bestString = result.bestTranscription.formattedString
                self.detectText.text = bestString
              
/
/
/
/
/
/
            } else if let error = error {
                self.showAlert(title: "SpeechNote", message: "There has been a speech recognition error.", handler: nil)
                print(error)
            }
        })
    }
  
    /
    func requestSpeechAuthorization() {
        SFSpeechRecognizer.requestAuthorization { authStatus in
            OperationQueue.main.addOperation {
                switch authStatus {
                case .authorized:
                    self.startButton.isEnabled = true
                case .denied:
                    self.startButton.isEnabled = false
                    self.detectText.text = "User denied access to speech recognition"
                case .restricted:
                    self.startButton.isEnabled = false
                    self.detectText.text = "Speech recognition restricted on this device"
                case .notDetermined:
                    self.startButton.isEnabled = false
                    self.detectText.text = "Speech recognition not yet authorized"
                }
            }
        }
    }
}