I normally use [plugin_url()][1] method to achieve enqueue. 

	function Zumper_widget_enqueue_script()
	{   
	wp_enqueue_script( 'my_custom_script', plugins_url('js/jquery.repeatable.js', __FILE__ ), '1.0.0', false );
	}
	add_action('admin_enqueue_scripts', 'Zumper_widget_enqueue_script');


  [1]: https://codex.wordpress.org/Function_Reference/plugins_url