WordPress needs to store uploads, plugins, and theme assets inside the wp-content folder. When the server cannot create or modify files there, WordPress stops the operation and shows an error. The failure occurs because the operating system blocks write access for the web‑server process.
Typical blockers include wrong file permissions, incorrect ownership, missing directories, or server policies such as SELinux or open_basedir. Even with correct permissions, a full disk or a mis‑configured upload_path constant will produce the same symptom.