Error in crystal report

SqlConnection conn = new SqlConnection(ConnectionString);
SqlCommand command = new SqlCommand();


String S4 = " select Period.Per,Account.Acc,GL.Deb,GL.Cre from Period,Account,GL where GL.Period=Period.Per and Account.AccLink=GL.AccLink and Month(Per) = '" + combo_Month.SelectedItem.ToString().Trim() + "' and Year(PerDate) ='" + combo_Year.SelectedItem.ToString().Trim() + "'";
SqlDataAdapter da4 = new SqlDataAdapter(S4, ConnectionString);
da4.Fill(a, "Dt_Tb_2");

String S6 = "select Period.Per from Period where Month(Date) = '" + combo_Month.SelectedItem.ToString().Trim() + "' and Year(Date) ='" + combo_Year.SelectedItem.ToString().Trim() + "'";
SqlDataAdapter da6 = new SqlDataAdapter(S6, ConnectionString);
DataTable dt = new DataTable();
da6.Fill(dt);

String S5 = "select Period.Date,Account.Acc,SUM(GL.Deb),SUM(GL.Cred) from Period,Account,GL where GL.Period=Period.Per and Account.AccLink=PostGL.AccLink and Per<='" + dt.Rows[0].ItemArray[0] + "' group by Period.Date,Account.Account";
SqlDataAdapter da5 = new SqlDataAdapter(S5, ConnectionString);
da5.Fill(a, "Dt_Tb_3");

frmTBViwer tb = new frmTBViwer(this);
tb.Show();