Sales Order Fulfilment - WebServices

Hi ,

We are getting an error in salesorder fulfillment ,the code which has been working fine from a long time. we got an error as

"You do not have permissions to set a value for element options due to one of the following reasons: 1) The field is read-only; 2) An associated feature is disabled; 3) The field is available either when a record is created or updated, but not in both cases."

We are using the following code
      this.login(true);
        ItemFulfillment ifm = new ItemFulfillment();
        ItemFulfillmentItemList ifml = new ItemFulfillmentItemList();
        ItemFulfillmentItem[] ifmitem = new ItemFulfillmentItem[1];
            RecordRef rff1 = new RecordRef();
            rff1.internalId = "1998"; //Internal id of salesorder from netsuite