https://www.ads-software.com/themes/wp-bootstrap-starter/

Then please ask in that theme’s support sub-forum.

https://www.ads-software.com/support/theme/wp-bootstrap-starter/#new-post

() is missing after get_template_directory_uri.

function add_popper() {
wp_enqueue_script('popper-js', get_template_directory_uri() . '/js/node_modules/popper.js/dist/popper.js', array(), '1.14.3', true);
}
add_action( 'wp_enqueue_scripts', 'add_popper' );