On Android 8.1 API 27 notification does not display

If you get this error should be paid attention to 2 items and them order:

1. NotificationChannel mChannel = new NotificationChannel(id, name, importance);
2. builder = new NotificationCompat.Builder(context, id);

Also NotificationManager notifManager and NotificationChannel mChannel are created only once.

There are required setters for Notification:

• builder.setContentTitle() // required
• .setSmallIcon() // required
• .setContentText() // required

See example:

private NotificationManager notifManager;
public void createNotification(String aMessage, Context context) {
    final int NOTIFY_ID = 0; // ID of notification
    String id = context.getString(R.string.default_notification_channel_id); // default_channel_id
    String title = context.getString(R.string.default_notification_channel_title); // Default Channel
    Intent intent;
    PendingIntent pendingIntent;
    NotificationCompat.Builder builder;
    if (notifManager == null) {
        notifManager = (NotificationManager)context.getSystemService(Context.NOTIFICATION_SERVICE);
    }
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
        int importance = NotificationManager.IMPORTANCE_HIGH;
        NotificationChannel mChannel = notifManager.getNotificationChannel(id);
        if (mChannel == null) {
            mChannel = new NotificationChannel(id, title, importance);
            mChannel.enableVibration(true);
            mChannel.setVibrationPattern(new long[]{100, 200, 300, 400, 500, 400, 300, 200, 400});
            notifManager.createNotificationChannel(mChannel);
        }
        builder = new NotificationCompat.Builder(context, id);
        intent = new Intent(context, MainActivity.class);
        intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP | Intent.FLAG_ACTIVITY_SINGLE_TOP);
        pendingIntent = PendingIntent.getActivity(context, 0, intent, 0);
        builder.setContentTitle(aMessage)                            // required
               .setSmallIcon(android.R.drawable.ic_popup_reminder)   // required
               .setContentText(context.getString(R.string.app_name)) // required
               .setDefaults(Notification.DEFAULT_ALL)
               .setAutoCancel(true)
               .setContentIntent(pendingIntent)
               .setTicker(aMessage)
               .setVibrate(new long[]{100, 200, 300, 400, 500, 400, 300, 200, 400});
    }
    else {
        builder = new NotificationCompat.Builder(context, id);
        intent = new Intent(context, MainActivity.class);
        intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP | Intent.FLAG_ACTIVITY_SINGLE_TOP);
        pendingIntent = PendingIntent.getActivity(context, 0, intent, 0);
        builder.setContentTitle(aMessage)                            // required
               .setSmallIcon(android.R.drawable.ic_popup_reminder)   // required
               .setContentText(context.getString(R.string.app_name)) // required
               .setDefaults(Notification.DEFAULT_ALL)
               .setAutoCancel(true)
               .setContentIntent(pendingIntent)
               .setTicker(aMessage)
               .setVibrate(new long[]{100, 200, 300, 400, 500, 400, 300, 200, 400})
               .setPriority(Notification.PRIORITY_HIGH);
    }
    Notification notification = builder.build();
    notifManager.notify(NOTIFY_ID, notification);
}