The requested payment method is not available - SOAP V2

[addmers]

I have a php code:

I'm using SOAP V2

$paymentMethod =  array(
   'method' => 'paypal_standard'
);

// add payment method
$result = $this->client->shoppingCartPaymentMethod($this->session,$this->cartID, $paymentMethod);

var_dump($result);

Error: The requested payment method is not available
Please advice on how to fix this.