Can not create sales order using Page id 42 Web Service

Hi All,

           I can't create sales order using page id 42 web service. I am trying to create from asp.net C#.

When I add item with Price list , the following error is occured. But when Item is not in Price list, it can insert successfull.

Error is 

Field SalesPriceExist is readonly!

Please give some suggestions. I m very new to Navision.

Thanks Advance.