Remove the “defer” in the line where the fullcalendar core component is being loaded - for example:
<script src="https://unpkg.com/@fullcalendar/core@4.4.2/main.min.js" defer></script>
…so that it looks like:
<script src="https://unpkg.com/@fullcalendar/core@4.4.2/main.min.js"></script>
Then it should work.