Laravel Class 'App\Http\Controllers\GuzzleHttp\Client' not found

You're going to want to get acquainted with PHP namespaces.

Most files in Laravel are namespaced. Calls to functions within a namespace start within that namespace, with two exceptions:

If you start the class name with a \, that tells PHP to start at the root-level namespace:

$client = new \GuzzleHttp\Client(); 

Or, you can put:

use GuzzleHttp\Client;

at the top of the file (you'll see a lot of these already throughout Laravel's default files) and then do

$client = new Client();

You might have not installed guzzle.

Run composer require guzzlehttp/guzzle to install it