The error comes from Redux Framework plugin. The main reason for that is your website can’t load resources from CDN (from a remote host). We can’t say exactly why it is happening for you. Maybe the remote host was down by the time you tried to do this or maybe it is a problem in your connection speed or server limitations.
As a workaround, you may place your CSS to the style.css file in the child theme. If you don’t have child theme you can find it in a downloaded package from themeforest.
Regards