Hi akhter,
Check this example. Now please take its reference and correct your code.
HTML
<table>
    <tr>
        <td>
            <asp:TextBox ID="txtremarks" runat="server" Width="454px"></asp:TextBox>
        </td>
    </tr>
    <tr>
        <td>Payment Mode              
            <asp:DropDownList ID="ddlpaymode" CssClass="form-control js-example-placeholder-single" Width="150px" runat="server">
                <asp:ListItem Value="Bank">Bank</asp:ListItem>
                <asp:ListItem Value="Cash">Cash</asp:ListItem>
                <asp:ListItem Value="Cash By Hand">Cash By Hand</asp:ListItem>
            </asp:DropDownList>
        </td>
    </tr>
    <tr>
        <td>Bank              
            <asp:DropDownList ID="ddlbank" CssClass="form-control js-example-placeholder-single" Width="200px" runat="server">
                <asp:ListItem Value="1">SBI</asp:ListItem>
                <asp:ListItem Value="2">AXIS</asp:ListItem>
                <asp:ListItem Value="3">HDFC</asp:ListItem>
            </asp:DropDownList>
        </td>
    </tr>
    <tr>
        <td >Location
            <asp:DropDownList ID="ddlloc" CssClass="form-control js-example-placeholder-single" Width="200px" runat="server">
                <asp:ListItem Value="1">Andheri</asp:ListItem>
                <asp:ListItem Value="2">Malad</asp:ListItem>
                <asp:ListItem Value="3">Goregaon</asp:ListItem>
            </asp:DropDownList>
        </td>
    </tr>
    <tr>
        <td>Cheque No:
        <asp:TextBox ID="txtchecqno" runat="server"></asp:TextBox>
        </td>
    </tr>
    <tr>
        <td>Date:
            <asp:TextBox ID="txtdate" TextMode="Date" runat="server"></asp:TextBox>
            <br />
        </td>
    </tr>
    <tr>
        <td>
            <asp:Button ID="btn_add" runat="server" Text="Add" OnClick="btn_add_Click" Width="62px" />
        </td>
    </tr>
    <tr>
        <td>
            <asp:GridView ID="gvv" runat="server" AutoGenerateColumns="False" DataKeyNames="Index" BackColor="White"
                BorderColor="#E7E7FF" BorderStyle="None" BorderWidth="1px" CellPadding="3" CssClass="myGridClass"
                Font-Names="Arial" GridLines="Horizontal" Height="198px" Style="margin-left: 3px" Width="807px">
                <AlternatingRowStyle BackColor="#F7F7F7" />
                <Columns>
                    <asp:TemplateField HeaderText="Sr No" HeaderStyle-Width="5%" HeaderStyle-HorizontalAlign="Left">
                        <ItemTemplate>
                            <%# Container.DataItemIndex + 1 %>
                        </ItemTemplate>
                    </asp:TemplateField>
                    <asp:TemplateField HeaderText="Payment_Mode">
                        <ItemTemplate>
                            <asp:Label ID="Payment_Mode" runat="server" Text='<%#Bind("Payment_Mode")%>'></asp:Label>
                        </ItemTemplate>
                    </asp:TemplateField>
                    <asp:TemplateField HeaderText="BankID" Visible="false">
                        <ItemTemplate>
                            <asp:Label ID="Bank_ID" runat="server" Text='<%#Bind("Bank_ID")%>'></asp:Label>
                        </ItemTemplate>
                    </asp:TemplateField>
                    <asp:TemplateField HeaderText="Bank" Visible="true">
                        <ItemTemplate>
                            <asp:Label ID="Bank_Name" runat="server" Text='<%#Bind("Bank_Name")%>'></asp:Label>
                        </ItemTemplate>
                    </asp:TemplateField>
                    <asp:TemplateField HeaderText="LocID" Visible="false">
                        <ItemTemplate>
                            <asp:Label ID="Loc_ID" runat="server" Text='<%#Bind("Loc_ID")%>'></asp:Label>
                        </ItemTemplate>
                    </asp:TemplateField>
                    <asp:TemplateField HeaderText="Location" Visible="true">
                        <ItemTemplate>
                            <asp:Label ID="Loc_Name" runat="server" Text='<%#Bind("Loc_Name")%>'></asp:Label>
                        </ItemTemplate>
                    </asp:TemplateField>
                    <asp:TemplateField HeaderText="Cheque_NO">
                        <ItemTemplate>
                            <asp:Label ID="Cheque_NO" runat="server" Text='<%#Bind("Cheque_NO")%>'></asp:Label>
                        </ItemTemplate>
                    </asp:TemplateField>
                    <asp:TemplateField HeaderText="Date">
                        <ItemTemplate>
                            <asp:Label ID="Chq_Date" runat="server" Text='<%#Bind("Chq_Date")%>'></asp:Label>
                        </ItemTemplate>
                    </asp:TemplateField>
                    <asp:CommandField ShowDeleteButton="True" ButtonType="Button" />
                </Columns>
                <FooterStyle BackColor="#B5C7DE" ForeColor="#4A3C8C" />
                <HeaderStyle BackColor="#4A3C8C" Font-Bold="True" ForeColor="#F7F7F7" />
                <PagerStyle BackColor="#E7E7FF" ForeColor="#4A3C8C" HorizontalAlign="Right" />
                <RowStyle BackColor="#E7E7FF" ForeColor="#4A3C8C" />
                <SelectedRowStyle BackColor="#738A9C" Font-Bold="True" ForeColor="#F7F7F7" />
                <SortedAscendingCellStyle BackColor="#F4F4FD" />
                <SortedAscendingHeaderStyle BackColor="#5A4C9D" />
                <SortedDescendingCellStyle BackColor="#D8D8F0" />
                <SortedDescendingHeaderStyle BackColor="#3E3277" />
            </asp:GridView>
        </td>
    </tr>
</table>
Code
C#
protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        if (ViewState["dt"] == null)
        {
            DataTable dt = new DataTable();
            dt.Columns.Add("Index");
            dt.Columns.Add("Payment_Mode");
            dt.Columns.Add("Bank_ID");
            dt.Columns.Add("Bank_Name");
            dt.Columns.Add("Loc_ID");
            dt.Columns.Add("Loc_Name");
            dt.Columns.Add("Chq_Date");
            dt.Columns.Add("Cheque_NO");
            ViewState["dt"] = dt;
        }
    }
}
protected void btn_add_Click(object sender, EventArgs e)
{
    if (ddlpaymode.SelectedItem != null && ddlbank.SelectedItem != null)
    {
        // int codeitem = Convert.ToInt32(DDLitem.SelectedItem.Value.ToString());
        string Descriptionitem = ddlpaymode.SelectedItem.Text.ToString();
        int bank_ID = Convert.ToInt32(ddlbank.SelectedItem.Value.ToString());
        string Bank_Name = ddlbank.SelectedItem.Text.ToString();
        int loc_ID = Convert.ToInt32(ddlloc.SelectedItem.Value.ToString());
        string Loc_Name = ddlloc.SelectedItem.Text.ToString();
        DataTable dtgridview = ViewState["dt"] as DataTable;
        DataRow dr2 = dtgridview.NewRow();
        bool ifExist = false;
        if (!ifExist)
        {
            dr2["Index"] = dtgridview.Rows.Count + 1;
            dr2["Payment_Mode"] = ddlpaymode.Text;
            dr2["Bank_ID"] = bank_ID;
            dr2["Bank_Name"] = Bank_Name;
            dr2["Loc_ID"] = loc_ID;
            dr2["Loc_Name"] = Loc_Name;
            dr2["Chq_Date"] = txtdate.Text;
            dr2["Cheque_NO"] = txtchecqno.Text;
            //  dr2["Time"] = DateTime.Now.ToString("HH:mm:ss");
            dtgridview.Rows.Add(dr2);
            DataView dv = dtgridview.DefaultView;
            // dv.Sort = "Time Desc";
            ViewState["dt"] = dtgridview;
            gvv.DataSource = dtgridview;
            gvv.DataBind();
            string remark = "";
            for (int i = 0; i < dtgridview.Rows.Count; i++)
            {
                string mode = dtgridview.Rows[i]["Payment_Mode"].ToString();
                string bank = dtgridview.Rows[i]["Bank_Name"].ToString();
                string cheque = dtgridview.Rows[i]["Cheque_NO"].ToString();
                remark += mode + "," + bank + ",";
                if (!string.IsNullOrEmpty(cheque))
                {
                    remark += cheque + ",";
                }
                remark += "    ";
            }
            txtremarks.Text = remark;
        }
    }
}
VB.Net
Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
    If Not IsPostBack Then
        If ViewState("dt") Is Nothing Then
            Dim dt As DataTable = New DataTable()
            dt.Columns.Add("Index")
            dt.Columns.Add("Payment_Mode")
            dt.Columns.Add("Bank_ID")
            dt.Columns.Add("Bank_Name")
            dt.Columns.Add("Loc_ID")
            dt.Columns.Add("Loc_Name")
            dt.Columns.Add("Chq_Date")
            dt.Columns.Add("Cheque_NO")
            ViewState("dt") = dt
        End If
    End If
End Sub
Protected Sub btn_add_Click(ByVal sender As Object, ByVal e As EventArgs)
    If ddlpaymode.SelectedItem IsNot Nothing AndAlso ddlbank.SelectedItem IsNot Nothing Then
        Dim Descriptionitem As String = ddlpaymode.SelectedItem.Text.ToString()
        Dim bank_ID As Integer = Convert.ToInt32(ddlbank.SelectedItem.Value.ToString())
        Dim Bank_Name As String = ddlbank.SelectedItem.Text.ToString()
        Dim loc_ID As Integer = Convert.ToInt32(ddlloc.SelectedItem.Value.ToString())
        Dim Loc_Name As String = ddlloc.SelectedItem.Text.ToString()
        Dim dtgridview As DataTable = TryCast(ViewState("dt"), DataTable)
        Dim dr2 As DataRow = dtgridview.NewRow()
        Dim ifExist As Boolean = False
        If Not ifExist Then
            dr2("Index") = dtgridview.Rows.Count + 1
            dr2("Payment_Mode") = ddlpaymode.Text
            dr2("Bank_ID") = bank_ID
            dr2("Bank_Name") = Bank_Name
            dr2("Loc_ID") = loc_ID
            dr2("Loc_Name") = Loc_Name
            dr2("Chq_Date") = txtdate.Text
            dr2("Cheque_NO") = txtchecqno.Text
            dtgridview.Rows.Add(dr2)
            Dim dv As DataView = dtgridview.DefaultView
            ViewState("dt") = dtgridview
            gvv.DataSource = dtgridview
            gvv.DataBind()
            Dim remark As String = ""
            For i As Integer = 0 To dtgridview.Rows.Count - 1
                Dim mode As String = dtgridview.Rows(i)("Payment_Mode").ToString()
                Dim bank As String = dtgridview.Rows(i)("Bank_Name").ToString()
                Dim cheque As String = dtgridview.Rows(i)("Cheque_NO").ToString()
                remark += mode & "," & bank & ","
                If Not String.IsNullOrEmpty(cheque) Then
                    remark += cheque & ","
                End If
                remark += "    "
            Next
            txtremarks.Text = remark
        End If
    End If
End Sub