How to add select2 js in wordpress admin side theme options

Use wp_enqueue_script function for including scripts. Also remove your jquery call. Use dependencies.

See an example

Place this code in functions.php of your theme or in main plugin file:

function enqueue_select2_jquery() {    wp_register_style( 'select2css', '//cdnjs.cloudflare.com/ajax/libs/select2/3.4.8/select2.css', false, '1.0', 'all' );    wp_register_script( 'select2', '//cdnjs.cloudflare.com/ajax/libs/select2/3.4.8/select2.js', array( 'jquery' ), '1.0', true );    wp_enqueue_style( 'select2css' );    wp_enqueue_script( 'select2' );}add_action( 'admin_enqueue_scripts', 'enqueue_select2_jquery' );

This action will include select2 libraries and jquery. Third parameter of wp_register_script told WordPress that for this script working fine should be included jquery core. See more details on developers portal. If set 3rd param to array('jquery') jquery core will be included before your select2 plugin automatically.

Than in your page, header, scrtip file (where you wanna) place JS with call select2 plugin:

<script type="text/javascript">  $(document).ready(function($) {      $('.option-tree-ui-select').select2();  });</script>