what is wrong with the code ?

You are changing variable sql to a new SQL statement. But you do not change SqlCommand.CommandText ?

if (rdbAdd.Checked == true)
{
    if (txtBarcode.Text.Length > 7)
    {
        txtBarcode.Text = txtBarcode.Text.Remove(0, 7);
        txtBarcode.SelectionStart = txtBarcode.Text.Length;
    }
    if (txtBarcode.Text.Length == 7)
    {
        sql = "select amount from storedData where barcode='" + txtBarcode.Text + "'";
        cmd = new SqlCommand(sql, con);

        amount = 0;
        con.Open();
        reader = cmd.ExecuteReader();
        try
        {
            reader.Read();
            if (reader.Read() == true)
            {
                amount = (int)reader["Amount"];
                amount++;
                sql = "update storedData set amount='" + amount + "' where barcode='" + txtBarcode.Text + "'";
                cmd.CommandText = sql;
                cmd.ExecuteNonQuery();
            }
            else
            {
                MessageBox.Show("new barcode add first");
            }
        }
        catch (SqlException ex)
        {
            MessageBox.Show(ex.ToString());
        }
        con.Close();
    }
}

But it will proberly fail any way. As long you have are running a ExecuteReader(), you cannot use the same SqlConnection for other SQL statements.