Uncaught Error: Interface 'PayPalHttp\Environment' not found

[Alberto123456]

I'am useing the https://github.com/paypal/Checkout-PHP-SDK

trying the first lines of the samples gives still an error.

$clientId = "<<PAYPAL-CLIENT-ID>>";
$clientSecret = "<<PAYPAL-CLIENT-SECRET>>";

$environment = new SandboxEnvironment($clientId, $clientSecret);
$client = new PayPalHttpClient($environment);

there is nowhere an interface Environment defined. How and where to do that?