1) Make sure you have an up-to-date theme version.
2) Please try to apply 755 or 777 CHMOD to:
…/wp-content/uploads/wp-less/THEME_NAME/css/ folder and files inside.
Then you need to navigate to Theme Options -> General and click “Save Options” button. Warning message will not bother you after that.
3) Please check upload path is default (in Settings > Media or in WP-config.php)