The cache directory is not writable by the PHP process. The operating system blocks file creation because the folder mode is too restrictive or the owner does not match the web‑server user. Security modules such as SELinux or suEXEC may also deny write access even when the numeric mode looks correct.
When the folder cannot be written, caching plugins fail to store generated files. The site loses the performance benefits of static cache and may show broken images or other generated assets.