select2 ruby on rails 5 does not work

application.css.scss

    *= require toastr
    *= require jquery-ui
    *= require select2
    *= require select2-bootstrap
    *= require_tree .
    *= require_self
    */



    @import 'bootstrap-sprockets';
    @import 'bootstrap';
    @import 'bootstrap-datetimepicker';
    @import "select2";
    @import "select2-bootstrap";
    @import "font-awesome-sprockets";
    @import "font-awesome";

application.js

    //= require jquery
    //= require turbolinks
    //= require jquery_ujs
    //= require jquery-ui/widgets/autocomplete
    //= require autocomplete-rails
    //= require select2-full
    //= require bootstrap-sprockets
    //= require moment
    //= require moment/es
    //= require bootstrap-datetimepicker
    //= require toastr
    //= require pickers
    //= require_tree .




    $( "#dropdown" ).select2({
        theme: "bootstrap"
     });

meetings.js

  $(document).ready(function() {
    $('#ajax-example').select2({
      ajax: {
        url: "/reuniones.json",
        dataType: "json",
        results: function(data, page) {
          return { 
            results: $.map( data, function(reunion, i) { 
              return { id: reunion.id, text: reunion.reunionfullnombre } 
            } )
          }
        }
      }
    });
  });

index.html.erb

(current index)

   <input type="text"  id="ajax-example" />

meetings_controller.rb

 def index
   @reuniones  = Reunion.order('reunionfullnombre').all  
 end

Here I would like the page to work.

Gemfile

 gem 'bootswatch-rails'
 gem 'simple_form'
 gem 'bootstrap-sass', '~> 3.3', '>= 3.3.7'
 gem 'jquery-rails'
 gem 'will_paginate', github: 'jonatack/will_paginate'
 gem 'ransack'
 gem 'will_paginate-bootstrap'
 gem "select2-rails"

_reunion.json.jbuilder

 json.extract! reunion, :id, :hora_pedido,:nombre ,:fecha_pedido, :hora_inicio, :hora_final, :fecha_entrega, :observacion, :subtotal, :planta_id, :centro_costo_id, :created_at, :updated_at

  json.url reunion_url(reunion, format: :json)

that what I get thanks for the help I've searched a lot and I'm new to this in rails just learn thanks.