We are going to make WordPress menu int bootstrap menu by walker class. first define class and called walker following code in function.php file

class submenu_wrapper extends Walker_Nav_Menu {
// add classes to ul sub-menus
function start_lvl( &$output, $depth = 0, $args = array() ) {
// depth dependent classes
$indent = ( $depth > 0 ? str_repeat( "\t", $depth ) : '' ); // code indent
$display_depth = ( $depth + 1); // because it counts the first submenu as 0
$classes = array(
/* 'dropdown-menu left-1'*/
'dropdown-menu'
);
$class_names = implode( ' ', $classes );   
 // build html
    if ($display_depth == 1) {
        $output .= "\n" . $indent . '<ul class="' . $class_names . '">' . "\n";
    } else {
        $output .= "\n" . $indent . '<ul class="' . $class_names . '">' . "\n";
    }
}
}

After the extending walker class, we will add classes and attribute in anchor tag by “nav_menu_link_attributes” Filter. for a you have to use it as per following code.

add_filter( 'nav_menu_link_attributes', 'anchor_menu_add_class', 10, 4 );    
function anchor_menu_add_class( $atts, $item, $args ,$depth=0) {
        if ($args->walker->has_children==1 && $depth === 0 ) {
          $atts['href']          = ! empty( $item->url ) ? $item->url : '';
          $atts['data-toggle']   = 'dropdown';
          $atts['class']         = 'dropdown-toggle';
          $atts['aria-haspopup']    = 'true';
          $atts['aria-expanded']='false';
        } elseif($depth === 0) {
            $atts['href'] = ! empty( $item->url ) ? $item->url : '';
            $atts['class']         = '';
        } elseif($depth === 1) {
          $atts['href'] = ! empty( $item->url ) ? $item->url : '';
          $atts['class']         = 'dropdown-item';
        }
        /*if($depth === 0) {
            $atts['href'] = ! empty( $item->url ) ? $item->url : '';
            $atts['class']         = 'nav-link';
        }elseif ($args->walker->has_children==1 && $depth === 0 ) {
          $atts['href']          = ! empty( $item->url ) ? $item->url : '';
          $atts['data-toggle']   = 'dropdown';
          $atts['class']         = 'nav-link dropdown-toggle';
          $atts['aria-haspopup']    = 'true';
          $atts['aria-expanded']='false';
        } elseif($depth === 1) {
          $atts['href'] = ! empty( $item->url ) ? $item->url : '';
          $atts['class']         = 'dropdown-item';
        }*/
       // print_r($args->walker->has_children);
        //echo "<pre>";
       // print_r($item);
        //die;
        return $atts;
    }

Calling walker In header.php

<?php 
echo wp_nav_menu(array('menu'=>'Home Menu','menu_class'=>'navbar-nav ml-auto','walker'=>new submenu_wrapper,'container'=>false)); 
?>