Composer and Google Auth JSON Key - php

Question

I need a sample code that shows me how I use the .json file I got from Google to authenticate that I am the owner of the storage Bucket.
Basically users upload their profile picture and I want to store it using Google Cloud, but every time I run the code below it says
Could not load the default credentials
I have gone to the URL it gives and I get a 404 error.
My index file has this in it
<?php
require 'vendor/autoload.php';
require 'sys/v1/core.php';
require 'sys/v1/google_cloud_storeage_upload.php';
$client = new Google_Client();
$client->setAuthConfig('/home/SITE/FOLDER/KEY-GOOGLE.json');
// $service implements the client interface, has to be set before auth call
$service = new Google_AnalyticsService($client);
if (isset($_GET['logout'])) { // logout: destroy token
unset($_SESSION['token']);
die('Logged out.');
}
if (isset($_GET['code'])) { // we received the positive auth callback, get the token and store it in session
$client->authenticate();
$_SESSION['token'] = $client->getAccessToken();
}
if (isset($_SESSION['token'])) { // extract token from session and configure client
$token = $_SESSION['token'];
$client->setAccessToken($token);
}
if (!$client->getAccessToken()) { // auth call to google
$authUrl = $client->createAuthUrl();
header("Location: ".$authUrl);
die;
}
?>