I can't get my output (simple audio frequency meter)

Hello, would you like to help me?
I want to try simple audio frequency meter project (https://www.arduino.cc/en/Tutorial/SimpleAudioFrequencyMeter)
But until now, I am not yet success to create it.
I am using samd21 compatible arduino zero from wemos, I has uploading the code to the arduino zero, but when I open serial monitor, there is not frequency detected as output.
What the cause of this error?

Please help me if you know some suggestion. :frowning:

The code is:

  Simple Frequency Meter for Arduino Zero

  Demonstrates how to sample an input signal and get back its frequency

  This example code is in the public domain


  created by Arturo Guadalupi <a.guadalupi@arduino.cc>
  10 Nov 2015

#include <AudioFrequencyMeter.h>

AudioFrequencyMeter meter;

void setup() {
  // put your setup code here, to run once:

  meter.setBandwidth(70.00, 1500);    // Ignore frequency out of this range
  meter.begin(A0, 45000);             // Intialize A0 at sample rate of 45kHz

void loop() {
  // put your main code here, to run repeatedly:
  float frequency = meter.getFrequency();
  if (frequency > 0)
    Serial.println(" Hz");