How do I change Select2 CSS when CodeIgniter form errors are present?

Good evening,

currently I'm working on a form which has a few Select2 dropdowns. The whole project is developted in CodeIgniter. Now I want to change the border color of the first Select2 if any CodeIgniter form errors are present, but somehow it doesn't work.

This doesn't work:

<?php if (validation_errors()) : ?>
    <script>
        $('.select2:first').css('border-bottom', '1px solid red');
    </script>
<?php endif; ?>

This doesn't work:

<?php if (validation_errors()) : ?>
    <script>
        $(document).ready(function() {
            $('.select2:first').css('border-bottom', '1px solid red');
        });
    </script>
<?php endif; ?>

This doesn't work:

<?php if (validation_errors()) : ?>
    <script>
        $(function() {
            $('.select2:first').css('border-bottom', '1px solid red');
        });
    </script>
<?php endif; ?>

Now, the funny thing is, if I execute $('.select2:first').css('border-bottom', '1px solid red'); in console it does change the color of the correct Select2. And if I put a alert("Somthing"); I get the alert window displayed, but it never changes the color.

Does anyone has an idea how I can solve this?