Fatal error: Class 'Webmozart\KeyValueStore\JsonFileStore' not found

after sending email via Mailgun i received this problem

Fatal error: Class 'Webmozart\KeyValueStore\JsonFileStore' not found

I use manual by Mailgun from their web page but i have problem with PULI package (probably). My code look like

require '../vendor/autoload.php';
use Mailgun\Mailgun;

# Instantiate the client.
$mgClient = new Mailgun('key-...');
$domain = "...mailgun.org";

# Make the call to the client.
$result = $mgClient->sendMessage($domain, array(
    'from'    => 'Digitalizacia',
    'to'      => '.....@gmail.com',
    'subject' => 'Kontakt Digitalizacia',
    'text'    => $_POST["email"]."\n".$_POST["subject"]."\n".$_POST["message"]
));
header("location: ../index.php#contact");

API key i changed and domain too.