Asset Optimization can’t find .css file

I loaded css file via child theme’s functions.php like this

add_action(‘wp_head’, ‘slick_css’);
function slick_css(){ ?>
<link rel=”stylesheet” type=”text/css” href=”https://mixing.co.kr/wp-content/themes/generatepress-child/js/slick/slick.css”/&gt;
<link rel=”stylesheet” type=”text/css” href=”https://mixing.co.kr/wp-content/themes/generatepress-child/js/slick/slick-theme.css”/&gt;
<?php }

but I can’t find slick-theme.css in asset optimization.
I’m using Manual mode. I’m experiencing issue that sometimes slick’s css is not loaded properly.

Thanks.