functions.php

<?php


function isirat_theme_styles() {

    wp_enqueue_style( 'bootstrap_css', get_template_directory_uri() . '/css/bootstrap.css' );
    wp_enqueue_style( 'main_css', get_template_directory_uri() . '/style.css' );

}

add_action( 'wp_enqueue_scripts', 'isirat_theme_styles');

function isirat_theme_js() {

    wp_enqueue_script( 'jquery', get_template_directory_uri() . '/js/jquery-3.2.1.min.js');
    wp_enqueue_script( 'bootstrap_js', get_template_directory_uri() . '/js/bootstrap.js' );



}

add_action( 'wp_enqueue_scripts', 'isirat_theme_js');


require_once('wp-bootstrap-navwalker.php');


register_nav_menus( array(
        'primary' => __( 'Primary Menu', 'isirat' ),
) );

header.php nav

 <nav class="navbar navbar-default" role="navigation">
            <div class="container">
                <!-- Brand and toggle get grouped for better mobile display -->
                <div class="navbar-header">
                <button type="button" class="navbar-toggle" data-toggle="collapse" data-target="#isiratnavigation">
                    <span class="sr-only">Toggle navigation</span>
                    <span class="icon-bar"></span>
                    <span class="icon-bar"></span>
                    <span class="icon-bar"></span>
                </button>
                <a class="navbar-brand" href="<?php echo home_url(); ?>">
                            <?php bloginfo('name'); ?>
                        </a>
                </div>

                    <?php
                        wp_nav_menu( array(
                            'menu'              => 'primary',
                            'theme_location'    => 'primary',
                            'depth'             => 2,
                            'container'         => 'div',
                            'container_class'   => 'collapse navbar-collapse',
                            'container_id'      => 'isiratnavigation',
                            'menu_class'        => 'nav navbar-nav',
                            'fallback_cb'       => 'WP_Bootstrap_Navwalker::fallback',
                            'walker'            => new WP_Bootstrap_Navwalker())
                        );
                    ?>
                </div>
            </nav>