Bootstrap CDN not loading

Please write your question in detail here.
(Example) I am creating a web application using Laravel on AWS Cloud9.
I pasted Bootstrap CDN to the specified location, but the function of Bootstrap is not implemented well. Where is the problem?

Applicable source code
<! DOCTYPE html>
<html lang = "{{app ()->getLocale ()}}">
    <meta charset = "utf-8">
    <meta http-equiv = "X-UA-Compatible" content = "IE = edge">
    <meta name = "viewport" content = "width = device-width, initial-scale = 1">
    <!-CSRF Token->
    <meta name = "csrf-token" content = "{{csrf_token ()}}">
    <title>{{config ('app.name', 'Laravel')}}</title>
    <link href = "{{asset ('css/app.css')}}" rel = "stylesheet">
    <link rel = "stylesheet" href = "https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.min.css">
                    <!-Collapsed Hamburger->
                    <button type = "button" data-toggle = "collapse" data-target = "# app-navbar-collapse">
                        <span>Toggle Navigation</span>
                    <!-Branding Image->
                        <!-{{config ('app.name', 'Laravel')}}->
                    <!-Left Side Of Navbar->
                    <!-Right Side Of Navbar->
                        <!-Authentication Links->
                        @if (Auth :: guest ())
                                    {{Auth :: user ()->name}}<span></span>
                                <ul role = "menu">
                                        <form action = "{{route ('logout')}}" method = "POST">
                                            {{csrf_field ()}}
        @yield ('content')
    <script src = "{{asset ('js/app.js')}}"></script>
      <script src = "https://code.jquery.com/jquery-3.3.1.slim.min.js" integrity = "sha384-q8i/X + 965DzO0rT7abK41JStQIAqVgRVzpbzo5smXKp4YfRvH + 8abtTE1Pi6jizo" crossorigin = "anonymous"></>
<script src = "https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.7/umd/popper.min.js" integrity = "sha384-UO2eT0CpHqdSJQ6hJty5KVphtPhzWj9WO1clHTMGa3JDZwrnQn4" F86dIHz0 = " script>
<script src = "https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/js/bootstrap.min.js" integrity = "sha384-JjSmVgyd0p3pXB1rRibZUAYoIIy6OrQ6VrjIEaFf/nJGzIxFDsf4x0xIM + B07jRM" crossorigin = ""
Supplemental information (FW/tool version etc.)

PHP 5.6.40
Laravel Framework 5.4.36