How to show a message of error when call a method inside of code behind from javascript

I´m using blur event, so i check the length of the textbox when the length is more than 25, the idea is show a error message, but so far i cant show the message.

page.ClientScript.RegisterStartupScript(page.GetType(), "SweetAlert", pMensaje, false)-> this is where i am supposed to show the message, right now i cant see anythig

<div class="grid3_1">
      <asp:Label runat="server" Text="Ubicación Exacta"></asp:Label>
       <asp:TextBox ID="txtDireciónExacta" CssClass="item19" style="resize:none" Height="100%" runat="server" Rows="5" TextMode="MultiLine"></asp:TextBox>
       <asp:Label runat="server" Text="#Plano"></asp:Label>
       <asp:TextBox ID="txtNumPlano"  CssClass="item21" runat="server"></asp:TextBox>
       <asp:Image runat="server" CssClass="item22"  ImageUrl="~/images/balloon_basic_blue.png" />
       <asp:Label runat="server" CssClass="item23" Text="Folio Real"></asp:Label>
       <asp:TextBox ID="txtFolioReal"  CssClass="item24" runat="server"></asp:TextBox>
       <asp:Label runat="server" CssClass="item25" Text="Area"></asp:Label>
       <asp:TextBox ID="txtArea" CssClass="item26" runat="server"></asp:TextBox>
        <asp:Label runat="server" CssClass="item27" Text="Finca Madre"></asp:Label>
       <asp:TextBox ID="txtFincaMadre" CssClass="item28" runat="server"></asp:TextBox>
       <asp:Label runat="server" CssClass="item29" Text="Tipo de Trámite"></asp:Label>
       <asp:DropDownList ID="droplistTipoTramite" CssClass="item30" runat="server" Height="30px">
        </asp:DropDownList>
       <asp:Label runat="server" CssClass="item31" Text="Servidumbre"></asp:Label>
       <asp:TextBox ID="txtServidumbre" CssClass="item32" runat="server"></asp:TextBox>
       </div>

<script>
window.onload = function () {
 let txtnumplano = document.getElementById('<%=txtNumPlano.ClientID%>');
  
 if (txtnumplano) {
     txtnumplano.addEventListener('blur', () => {
      PageMethods.LongitudTxtNumPlano(txtnumplano.textLength);
            });
   }


}
</script>

[System.Web.Services.WebMethod()]
[System.Web.Script.Services.ScriptMethod()]
public static void LongitudTxtNumPlano (string caracteres){
 pMensaje = vMensajes.mostrarMensaje("No empty", "error", "Atención");
 int count=int.Parse(caracteres);
           
 if (count> 25)
    {
        Page page = (Page)HttpContext.Current.CurrentHandler;
        page.ClientScript.RegisterStartupScript(page.GetType(), "SweetAlert", pMensaje, false);
    }
}