Google Oauth2 + Sheet API: error 401: invalid authentication credentials

So I follow the guide from https://developers.google.com/sheets/api/quickstart/php and https://developers.google.com/identity/protocols/OAuth2WebServer to run on my localhost. But i get this error:

Fatal error: Uncaught Google_Service_Exception: { "error": { "code": 401, >"message": "Request had invalid authentication credentials. Expected OAuth >2 access token, login cookie or other valid authentication credential. See >https://developers.google.com/identity/sign-in/web/devconsole-project.", >"errors": [ { "message": "Request had invalid authentication >credentials.", "reason": "authError" } ], "status": "UNAUTHENTICATED" }

Here is my files:

index.php

<?php
require_once __DIR__.'/vendor/autoload.php';

session_start();

$client = new Google_Client();
$client->setAuthConfig('client_secrets.json');
$client->setScopes(Google_Service_Sheets::SPREADSHEETS);



if (isset($_SESSION['access_token']) && $_SESSION['access_token']) {
    $client->setAccessToken($_SESSION['access_token']);
    $service = new Google_Service_Sheets($client);
    $spreadsheetId = '1BxiMVs0XRA5nFMdKvBdBZjgmUUqptlbs74OgvE2upms';
    $range = 'Class Data!A2:E';
    $response = $service->spreadsheets_values->get($spreadsheetId, $range);
    $values = $response->getValues();


    if (empty($values)) {
        print "No data found.\n";
    } 
    else 

    {
        print "Name, Major:\n";
        foreach ($values as $row) 
        {
            // Print columns A and E, which correspond to indices 0 and 4.
            printf("%s, %s\n", $row[0], $row[4]);
        }
    }

} 
else 
{
  $redirect_uri = 'http://' . $_SERVER['HTTP_HOST'] . '/oauth2callback.php';
  header('Location: ' . filter_var($redirect_uri, FILTER_SANITIZE_URL));
}

and

oauth2callback.php

<?php
require_once __DIR__.'/vendor/autoload.php';

session_start();

$client = new Google_Client();
$client->setAuthConfigFile('client_secrets.json');
$client->setRedirectUri('http://' . $_SERVER['HTTP_HOST'] . '/oauth2callback.php');
$client->setScopes(Google_Service_Sheets::SPREADSHEETS_READONLY);

if (! isset($_GET['code'])) {
  $auth_url = $client->createAuthUrl();
  header('Location: ' . filter_var($auth_url, FILTER_SANITIZE_URL));
} else {
  $client->authenticate($_GET['code']);
  $_SESSION['access_token'] = $client->getAccessToken();
  $redirect_uri = 'http://' . $_SERVER['HTTP_HOST'] . '/';
  header('Location: ' . filter_var($redirect_uri, FILTER_SANITIZE_URL));
}

Thank you!