Kavitha,
This is not an item number related issue. Something is missing in the header/line xml parameter. Please pass the value to the parameter "UpdateIfExists = 1" in the header <taIVCreateItemPriceListHeader> section for update process.
Kavitha,
This is not an item number related issue. Something is missing in the header/line xml parameter. Please pass the value to the parameter "UpdateIfExists = 1" in the header <taIVCreateItemPriceListHeader> section for update process.