The WP_Error: Could not create temporary file error occurs when WordPress attempts to write a temporary file and the operation fails. The failure usually stems from server‑level settings such as a missing or unwritable PHP upload_tmp_dir, incorrect folder permissions, open_basedir restrictions, or lack of disk space.
WordPress relies on a writable temporary directory to stage uploads, plugin packages, and core updates. When the directory is unavailable, PHP returns an error and WordPress propagates it as the WP_Error message.