CTools CSS cache error
That directory must be writable by the usre that runs the web server (apache, for example).

So you have to:


go to admin/config/media/file-system and check the path of public files (may be sites/default/files)
check which user runs web server (on Ubuntu could be "www-data", on centos/fedora/redhat could be "apache")
using a terminal go to the document root of drupal and set the owner of the directory: 



  sudo chown -R www-data:www-data sites/default/files (or apache:apache)



if you couldn't use a terminal, use the ftp client or contact your hosting provider


PS: this instruction works on *nix servers