string SQL = "SELECT fio_s, Sum(debit) WHERE dat_z between @ZaezdDate1 AND @ZaezdDate2 Group By fio_s";                          
using (OleDbConnection con = new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0;Data Source=My_BD2_1.mdb"))
{
   con.Open();
    OleDbCommand cmd = new OleDbCommand(SQL, con);
    cmd.Parameters.AddWithValue("@ZaezdDate1", dt1);
    cmd.Parameters.AddWithValue("@ZaezdDate2", dt2);
    OleDbDataAdapter da = new OleDbDataAdapter(cmd);
    DataSet ds = new DataSet();
    da.Fill(ds); //ошибка здесь
    dataGridView1.DataSource = ds.Tables[0];