This issue may be is due to the following errors:
1. User of server (e.g Apache, Nginx) can’t write file. Let chmod file style.css and config.less (all in the theme directory) is 777.
2. Memory of hosting is restricted. Please increase memory limit to 128M follow this way: http://codex.wordpress.org/Editing_wp-config.php#Increasing_memory_allocated_to_PHP or you have to increase memory for your hosting.
3. Your PHP version is outdated. We require PHP version 7.0 or higher.