Subject forum not allowing posts

When I try to post to Subject Forum, I get an error that says "unable to write to forum" (this was working perfectly last week, and nobody in the company has changed it - because we don't know how to!) If I follow the link for "further information", it tells me to turn on Debugging in Site Administration. But there is no Debugging in Site Administration. I've tried it from a different browser as well, both as plain text and with the files I wanted to include - same problem. It may only affect one course on our Moodle as another course is allowing posts on the forum.