The Redux theme options framework loads some files from an external CDN server rather than loading them from your own server to improve the page load speed. If the CDN server goes down and you will receive this error but it usually fixes itself very quickly. If the error message does not go away then your server may not allow CDN files to be loaded. If this is the case ask your host to enable this, but if they can't you will need to host the files yourself.
To do this download the following plugin at https://github.com/reduxframework/redux-vendor-support/archive/master.zip and install and active it on your site.
Now open lib/framework/theme-config.php and find:
'use_cdn' => true,
'use_cdn' => false,