Fatal error: Class 'Guzzle\Service\Builder\ServiceBuilder' not found

I installed your plugin today (including the required Amazon Web Services plugin) on a WP multisite install, I have added the required amazon S3 settings to wp-config.php but when I try and access the plugin settings page at /wp-admin/network/admin.php?page=amazon-s3-and-cloudfront I get this error:

Fatal error: Class ‘Guzzle\Service\Builder\ServiceBuilder’ not found in /[path_to_home]/public_html/wp-content/plugins/amazon-web-services/vendor/aws/Aws/Common/Aws.php on line 26

The plugin is not working, or at least it doesn’t seem like it’s working because I tried uploading media to the site and it is still not being served from Amazon S3.

Any ideas?

https://www.ads-software.com/plugins/amazon-s3-and-cloudfront/