Method Not Implemented message

Hi,
WP is installed on its own Linux server which I’ve been using for a month or so.

Before I realized that you couldn’t insert PHP directly into a post, I tried it and got this message:

Method Not Implemented
POST to /wp-admin/page.php not supported.

I’ve installed Exec-PHP (which is to up-to-date) and I still get the same message.

Does this sound like an issue with the PHP or something else?
Thanks very much in advance for any ideas!