This error may also mean that you have specified the wrong project ID and password. If this data belongs to a different payment method as intended by you, it is possible that the parameters for that other method are not the same as the ones you are sending. So please make sure that the project data you specify really match your payment method.
Another possible error cause is the format of your request. Make sure you send the content-type header as "application/x-www-form-urlencoded; charset=utf-8" and that the request data is formatted as "form URL encoded" (basically as if sent from an HTML form) and not in JSON format.
Please keep in mind that voids are only allowed for reserved transactions (type=AUTH). Normal ones (type=sale) may not be voided.