Problem solved guys thanks to Rajilesh Panoli,
from github link provided in the above comment.
download wp-bootstrap-navwalker.php and copy that file to your root directory of the theme and include that file in your functions.php file, then
use following code:
'fallback_cb' => 'WP_Bootstrap_Navwalker::fallback',
'walker' => new WP_Bootstrap_Navwalker(),
in the wp_nav_menu() function in the header.php file (or wherever you have used this function).