Saving error in web form

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ASP.NET

I am having a web form which has few text box and drop down controls. I enter all the required data and have to save in the database. when i am saving i am getting the following error "Sys.WebForms.PageRequestManagerServerErrorException:"
The save operation is not being done. Can anyone help me on this? PFA code.

<table style="width: 100%; border-color: #FFF; height: 40%" cellpadding="3" cellspacing="0">
                            <tr style="height: 30px">
                                <td>
                                    &nbsp;<asp:Label ID="lblName" runat="server" Text="Name:" ForeColor="Black"
                                        Font-Bold="True" Width="120px"></asp:Label>
                                </td>
                                <td>
                                    <label class="custom-select">
                                        <asp:DropDownList ID="ddlName" runat="server" DataTextField="NAME_DESC"
                                            DataValueField="NAME_ID">
                                        </asp:DropDownList>
                                    </label>
                                </td>
                                <td>
                                    &nbsp;<asp:Label ID="Label16" runat="server" Text="Package No:" ForeColor="Black"
                                        Font-Bold="True" Width="120px"></asp:Label>
                                </td>
                                <td>
                                    <label class="custom-select">
                                        <asp:DropDownList ID="ddlPkg" runat="server" DataTextField="PACKAGE_DESC"
                                            DataValueField="PACKAGE_ID" TabIndex="1">
                                        </asp:DropDownList>
                                    </label>
                                </td>
                            </tr>
                            <tr style="height: 30px">
                                <td>
                                    &nbsp;<asp:Label ID="Label2" runat="server" Text="Category:" ForeColor="Black"
                                        Font-Bold="True" Width="130px"></asp:Label>
                                </td>
                                <td>
                                    <label class="custom-select">
                                        <asp:DropDownList ID="ddlCat" runat="server" DataTextField="CATEGORY_DESC"
                                            DataValueField="CATEGORY_ID" TabIndex="2">
                                        </asp:DropDownList>
                                    </label>
                                </td>
                                <td>
                                    &nbsp;<asp:Label ID="Label4" runat="server" Text="Sub Task:" ForeColor="Black" Font-Bold="True"
                                        Width="130px"></asp:Label>
                                </td>
                                <td>
                                    <asp:TextBox ID="txtSubTask" runat="server" CssClass="textboxStyle" MaxLength="100"
                                        TextMode="MultiLine" Height="30px" Width="180px"></asp:TextBox>
                                </td>
                            </tr>
                        </table>
            <table>
                    <tr style="height: 35px">
                        <td align="center">
<asp:Button ID="btnSave" runat="server" Text="Save" Width="80px" CssClass="buttonStyle"
                                TabIndex="15" OnClick="btnSave_Click" />
 </td>
                    </tr>
                </table>

This is the error i am getting:

Uncaught Sys.WebForms.PageRequestManagerServerErrorException: Sys.WebForms.PageRequestManagerServerErrorException: An unknown error occurred while processing the request on the server. The status code returned from the server was: 0