Add classes to menus.php

/**
 * Add class to menu-item
 */
add_filter('nav_menu_css_class', function ($classes, $item) {
    $classes[] = 'nav-item';
    return $classes;
}, 10, 2);

/**
 * Add class to menu-link
 */
add_filter('nav_menu_link_attributes', function ($atts, $item, $args) {
    // check if the item is in the primary menu
    if ($args->theme_location == 'primary_navigation') {
        // add the desired attributes:
        $atts['class'] = 'nav-link';
    }
    return $atts;
}, 10, 3);