Add an ID to your script file so you can query it.
<script id="hljs" async src="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/9.0.0/highlight.min.js"></script>
Then add a load listener to it in JavaScript
<script>
var script = document.querySelector('#hljs');
script.addEventListener('load', function() {
hljs.initHighlightingOnLoad();
});
</script>