It would be good to see your code.  I have not seen this specific message before.

Hi n0tgunshy1 

Here's the part of code that I'm using. Please advice

using (DispatchObject oARCustomerEntry = new DispatchObject())
{
object[] nextSalesOrderNumber = new object[] { "SalesOrderNo$" };

//Getting next SalesOrder Number
oARCustomerEntry.InvokeMethodByRef("nGetNextSalesOrderNo", nextSalesOrderNumber);
objCreateNewSalesOrder.SalesOrderNo = nextSalesOrderNumber[0].ToString();
object retVal = 0;

retVal = oARCustomerEntry.InvokeMethodByRef("nSetKeyValue", new object[] { "SalesOrderNo$", objCreateNewSalesOrder.SalesOrderNo });
retVal = oARCustomerEntry.InvokeMethod("nSetKey");

retVal = oARCustomerEntry.InvokeMethodByRef("nSetValue", new object[] { "CustomerNo$", CustomerPONumber });
retVal = oARCustomerEntry.InvokeMethodByRef("nSetValue", new object[] { "ARDivisionNo$", ARDivisionNo });
// other fields

string PRODUCT_CODE__C = "10035";
string QUANTITY = "1";
string UNITPRICE = "15";

DispatchObject sales_order_line = new DispatchObject(oARCustomerEntry.GetProperty("oLines"));

string line = "000001";

object EditKey = sales_order_line.InvokeMethod("sGetEditKey", line);
retVal = sales_order_line.InvokeMethod("nEditLine", EditKey);

sales_order_line.InvokeMethod("nSetValue", "ItemCode$", PRODUCT_CODE__C);
sales_order_line.InvokeMethod("nSetValue", "UnitPrice", Convert.ToDecimal(UNITPRICE)); //NO $ sign
int qty = Convert.ToInt32(Convert.ToDecimal(QUANTITY));
sales_order_line.InvokeMethod("nSetValue", "QuantityOrdered", qty); //No $ sign

sales_order_line.InvokeMethod("nWrite");

retVal = oARCustomerEntry.InvokeMethod("nWrite");

if (retVal.ToString() == "0")
{
object errorMsg = oARCustomerEntry.GetProperty("sLastErrorMsg");
Console.WriteLine("Error Message: " + errorMsg.ToString());
}
else
{
Console.WriteLine("Success");
}
}

You said you are importing new orders, right?  If so, you don't want to use the EditLine method, you use the AddLine method and you don't have to assign a key.  I have no idea if this is causing that error because I've never seen it before, but it's definitely wrong for new orders.  Also, I would advise you to always set the line Quantity before the Unit Price, because setting the Quantity triggers a price calculation.