You are using
Content1
so assuming this is your child page. You master page already have FORM tag and you tried adding HTML again in your contents so getting more that 1 FORM error.
Try with below it should resolve your issue.
<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" runat="server">
<div>
<asp:Label ID="Label1" CssClass="label" runat="server" Text="User ID:" Width="160px"></asp:Label>
<asp:TextBox ID="idBox" CssClass="textbox" runat="server" Width="350px"></asp:TextBox>
<br />
<br />
<asp:Label ID="Label2" CssClass="label" runat="server" Text="User Name:" Width="160px"></asp:Label>
<asp:TextBox ID="nameBox" CssClass="textbox" runat="server" Width="350px"></asp:TextBox>
<br />
<br />
<asp:Label ID="Label3" CssClass="label" runat="server" Text="User Address:" Width="160px"></asp:Label>
<asp:TextBox ID="addressBox" CssClass="textbox" runat="server" Width="350px"></asp:TextBox>
<br />
<br />
<asp:Label ID="Label4" CssClass="label" runat="server" Text="User Contact:" Width="160px"></asp:Label>
<asp:TextBox ID="contactBox" CssClass="textbox" runat="server" Width="350px"></asp:TextBox>
<br />
<br />
<asp:Label ID="Label5" CssClass="label" runat="server" Text="User Email:" Width="160px"></asp:Label>
<asp:TextBox ID="emailBox" CssClass="textbox" runat="server" Width="350px"></asp:TextBox>
<br />
<br />
<asp:Label ID="Label6" CssClass="label" runat="server" Text="User DOB:" Width="160px"></asp:Label>
<asp:TextBox ID="dobBox" CssClass="textbox" runat="server" Width="350px"></asp:TextBox>
<br />
<br />
<asp:Button ID="insert" runat="server" CssClass="btn btn-primary" Text="Insert" OnClick="insertUser" Width="160px" />
<br />
<br />
</div>
<div class="table-responsive">
<asp:GridView ID="GridView1" CssClass="table" runat="server">
</asp:GridView>
</div>
</asp:Content>
<asp:Content ID="Content3" ContentPlaceHolderID="ContentPlaceHolder3" runat="server">
<form runat="server">
<div class="dropdown" aria-labelledby="messagesDropdown">
<ul class="dropdown-menu">
<li>
<asp:LinkButton ID="btnLogout" runat="server" OnClick="btnLogout_Click" Text="Logout"></asp:LinkButton></li>
<li>
<asp:LinkButton ID="changePassword" runat="server" PostBackUrl="~/ChangePassword.aspx" Text="Change Password"></asp:LinkButton></li>
</ul>
</div>
</asp:Content>