Adding PHP to posts

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

I’m trying to add PHP coding to a couple posts on my site. I’ve installed Exec-PHP which seems to be up to date. However, when I try to insert even simple code, I get:

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

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