Quite a lot of characters are illegal in for names: ? * \ and/ fit example.
And the default date time format on your system is likely to be dd/MM/yyyy or MM/dd/yyyy
So try this:
string namefile = "Stock_Flow_Report_2015" + DateTime.Now.ToString("dd-MM-yyyy");
Or better:
string namefile = DateTime.Now.ToString("yyyy-MM-dd") + "Stock_Flow_Report_2015;