Hi,
The DIV element in your panel is an HTML control so you cannot access it directly. To access it in code behind you need to decorate it with
runat="server" attribute.
<asp:Panel ID="PanelFL" runat="server" >
<asp:Button ID="chatFLclose" runat="server" Text="Close" />
<div id="DivChat" runat="server" > </div>
<asp:TextBox ID="txtChat" runat="server"></asp:TextBox>
<asp:Button ID="btnSend" runat="server" Text="Send" onclick="btnSend_Click" />
</asp:Panel>
and you can get access it now as below -
protected void btnSend_Click(object sender, EventArgs e)
{
DivChat.InnerText = txtChat.Text;
}