Your gallery's 'jbcore' folder is either in an incorrect location on your web server or has not been uploaded at all.
When manually embedding a Juicebox gallery in a CMS environment (such as Drupal), it can be difficult to know where to upload files and folders to (as the web pages are created dynamically by Drupal) so I would recommend that you embed your gallery using the baseUrl method of embedding as documented here. This method of embedding allows you to keep the gallery files inside the gallery folder and you would upload the entire gallery folder (not just the contents) to your web server. Also, it does not matter where on your web server you upload your gallery folder to as long as the paths within the embedding code (the path to the 'juicebox.js' file and the baseUrl itself, pointing towards the gallery folder) are correct.
As an example, if you have a gallery folder named 'my_gallery_folder' and you upload the entire gallery folder to your web site's root directory, then you would be able to use the following embedding code. (This code could be used in any page throughout your web site without modification. The leading slashes in the paths denote your root directory.)
<!--START JUICEBOX EMBED-->
<script src="/my_gallery_folder/jbcore/juicebox.js"></script>
<script>
new juicebox({
baseUrl: "/my_gallery_folder/",
containerId: "juicebox-container",
galleryWidth: "100%",
galleryHeight: "600",
backgroundColor: "#222222"
});
</script>
<div id="juicebox-container"></div>
<!--END JUICEBOX EMBED-->