I have problem with my modal id="cropper_js_banner" is not showing after clicking the attachment button. This is just part of the code. I just want to know why my cropper_js_banner is not showing after clicking the choosing the picture.

You are missing bootstrap, just add this script

<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.12.9/umd/popper.min.js" integrity="sha384-ApNbgh9B Y1QKtv3Rn7W3mgPxhU9K/ScQsAP7hUibX39j7fakFPskvXusvfa0b4Q" crossorigin="anonymous"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/js/bootstrap.min.js" integrity="sha384-JZR6Spejh4U02d8jOt6vLEHfe/JQGiRRSQQxSfFWpi1MquVdAyjUar5 76PVCmYl" crossorigin="anonymous"></script>

https://getbootstrap.com/docs/4.0/getting-started/introduction/