PayPal Standard Checkout is not generating an OAuth token (PHP)

I'm trying to implement PayPal Standard Checkout in PHP, and when I make a server-side request, this error appears:"Following authentication credentials are required:\n-> Client is not authorized. An OAuth token is needed to make API calls."

and the $client returns an empty array ({}). Can someone please help me?

However, the client_id and client_secret are correct.I have already run composer require paypal-server-sdk.

<?phprequire_once "vendor/autoload.php";use PaypalServerSdkLib\Authentication\ClientCredentialsAuthCredentialsBuilder;use PaypalServerSdkLib\Environment;use PaypalServerSdkLib\PaypalServerSDKClientBuilder;use PaypalServerSdkLib\Models\Builders\MoneyBuilder;use PaypalServerSdkLib\Models\Builders\OrderRequestBuilder;use PaypalServerSdkLib\Models\Builders\PurchaseUnitRequestBuilder;use PaypalServerSdkLib\Models\Builders\AmountWithBreakdownBuilder;use PaypalServerSdkLib\Models\Builders\ShippingDetailsBuilder;use PaypalServerSdkLib\Models\Builders\ShippingOptionBuilder;use PaypalServerSdkLib\Models\ShippingType;header("Access-Control-Allow-Origin: *"); header("Access-Control-Allow-Methods: GET, POST, OPTIONS"); header("Access-Control-Allow-Headers: Content-Type, Authorization");header("Access-Control-Allow-Credentials: true"); header('Content-Type: application/json');if ($_SERVER['REQUEST_METHOD'] === 'OPTIONS') {    http_response_code(200);    exit();}$PAYPAL_CLIENT_ID = getenv("PAYPAL_CLIENT_ID");$PAYPAL_CLIENT_SECRET = getenv("PAYPAL_CLIENT_SECRET");$client = PaypalServerSdkClientBuilder::init()    ->clientCredentialsAuthCredentials(        ClientCredentialsAuthCredentialsBuilder::init(            $PAYPAL_CLIENT_ID,            $PAYPAL_CLIENT_SECRET        )    )    ->environment(Environment::SANDBOX)    ->build();echo json_encode($client);

This code should return an OAuth token to allow the use of PayPal's createOrder and captureOrder methods.This code is in PayPal's documentation.