Wordfence tries to modify the .htaccess file to enable its firewall rules. The operation fails when the web‑server process cannot change the file. This happens because the file system blocks write access, the file is owned by a different user, or the server environment does not support .htaccess modifications.
The result is that Wordfence remains in learning mode and cannot provide full protection. The error appears during the firewall optimization step and shows a red notice in the WordPress admin.