MS Dynamics Web services - Update purchase order line validation error

Hello,

I'm trying to update some purchase order line fields by using this code:

public string UpdatePo(object itemKey, DateTime PromissedShipDate, DateTime PromissedDate, string Quantity, string UnitCost)
        {
            DynamicsGPClient client = new DynamicsGPClient();
            Context context = new Context();
            CompanyKey companyKey = new CompanyKey();
            companyKey.Id = (Convert.ToInt16(this.companyNo));
            context.OrganizationKey = (OrganizationKey)companyKey;

            // change
            ItemKey key = (ItemKey)itemKey;
            PurchaseOrderLine line = PurchaseOrder.Lines.Where(x => x.ItemKey.Id == key.Id).Select(x => x).FirstOrDefault();
            line.PromisedShipDate = PromissedShipDate;
            line.PromisedDate = PromissedDate;
            line.QuantityOrdered.Value = Convert.ToDecimal(Quantity);
            line.UnitCost.Value = Convert.ToDecimal(UnitCost);


            // save
            try
            {
                Policy purchaseOrderUpdatePolicy = client.GetPolicyByOperation("UpdatePurchaseOrder", context);
                client.UpdatePurchaseOrder(this.PurchaseOrder, context, purchaseOrderUpdatePolicy);
                return null;
            }
            catch (Exception ex)
            {
                return ex.Message;
            }
}

I am getting this exception:

A validation exception has occurred.
Validation Errors:
- WarehouseKey has not been assigned to ItemKey in Inventory.
- This budget or cost category does not allow an inventoried item                                                                                                                                                                                               
- Document Date is required for PA                                                                                                                                                                                                                              
- Unable to calculate PABBeginDate info                                                                                                                                                                                                                         
- A validation error has occurred.

Can somebody help me please? Thanks.

Ubi