warning in recorder prepare

Question

I have a audiorecording program on Android in C#, when I debugging program I have a warning on

recorder.Prepare (); // Prepared state (open failed: ENOENT)

a had read already many topics, but I don't understand because of what this warning. thank in advance

public class Recorder
{
    MediaRecorder recorder = null;

    public bool IsRecording { get { return recorder != null; } }

    public void Start ()
    {

        var filePath = Player.filePath;
        if (File.Exists (filePath)) {
            File.Delete (filePath);
        }
        if (recorder == null) {
            recorder = new MediaRecorder (); // Initial state.
        } else {
            recorder.Reset ();
        }
        recorder.SetAudioSource (AudioSource.Mic); 
        recorder.SetOutputFormat (OutputFormat.ThreeGpp);
        recorder.SetAudioEncoder (AudioEncoder.AmrNb);
        // Initialized state.
        recorder.SetOutputFile (filePath); 
        // DataSourceConfigured state.
        recorder.Prepare (); // Prepared state
        recorder.Start (); // Recording state.
    }
    public void Stop ()
    {
        if (recorder != null){
            recorder.Stop ();
            recorder.Release ();
            recorder = null;
        }
    }
}

}