Issue on Running Image upload on WP Metabox Custom Image Loader

I had the same error because I forgot to add:

wp_enqueue_media();

Then I found this how to and solved the problem like this:

1: You have to enqueue WP JavaScript library for media and your own JS file:

public function enqueue_scripts() {
  //WP library
  wp_enqueue_media();
  //Your JS file
  wp_register_script('name-of-your-script', plugin_dir_url( __FILE__ ) . 'js/path-to-your.js', array( 'jquery'), $this->version, true);
  wp_enqueue_script($this->plugin_name);
}
add_action('admin_enqueue_scripts', 'enqueue_scripts');

2: Create html for the form parts

<input class="img-01" type="text" name="img-01" size="100" />
<input type="button" class="button wk-button" data="img-01" value="<?php _e('Select image', 'your-text-domain');?>" />
<div><img src="" class="img-01"/></div>

3: Add code to your JS file (js/path-to-your.js)

jQuery(function ($) {
  $(document).ready(function () {
    $(".wk-button").click(function (e) {
      var wkMedia;
      // Using dynamic selector is useful if you want to use media selection/upload more then once within a single page (this value is populated from data attribute of the button
      var selector = $(this).attr("data");
      // Title is populated from value of the button to make it ready for localization
      var title = $(this).attr("value");
      e.preventDefault();
      // If the upload object has already been created, reopen the dialog
      if (wkMedia) {
        wkMedia.open();
        return;
      }
      // Extend the wp.media object
      wkMedia = wp.media.frames.file_frame = wp.media({
        title: title,
        button: {
          text: title,
        },
        multiple: false,
      });

      // When a file is selected, grab the URL and set it as the text field's value
      wkMedia.on("select", function () {
        var attachment = wkMedia.state().get("selection").first().toJSON();
          $("input." + selector).val(attachment.url);
          $("img." + selector).attr("src",attachment.url);        
      });
      // Open the upload dialog
      wkMedia.open();
    });
  });
});