Invalid placeholder character in C# for SQL query

Question

I was trying to execute the query. But somehow I'm getting "Invalid placeholder character" error.

public static DataTable GetOrderItem(TransactionContext tc, int challanID)
{
    string sql = string.Empty;

    sql = SQLParser.MakeSQL(@"Select oo.CashMemoNo CashMemoNo, o.Name OutletName,  o.Code OutletCode, o.Address Address, oo.ExpectedDeliveryDate ExpectedDeliveryDate,
                            '-', s.code SKUCode, s.Name SKUName, (ooi.IssuedQty /s.ConversionValue) QuantityCtn, (ooi.IssuedQty %  s.ConversionValue) Quantity,
                            ooi.FreeQty FreeQty, 'Ctn-Pcs', ooi.TradePrice TradePrice, 0, 15, 0, 'post', '-', 0, 'skuType', ooi.BatchNo BatchNo,
                            oo.SectionID section, oo.DSRID sso, oo.challanid challanNo
                            from outletorderitem ooi 
                            inner join outletorder oo on oo.outletorderid= ooi.outletorderid
                            inner join outlet o on o.outletid= oo.outletid 
                            inner join SKU s on s.SKUID= ooi.SKUID 
                            where oo.challanid = %n and ooi.FreeQty > 0 ", challanID);

    return tc.ExecuteDataSet(sql).Tables[0];
}