Write something first
The comment UI blocked submit because the textarea was empty. Client-only check before POST.
Why this happens
Same idea as Comment cannot be empty, but shown immediately in the form. If an empty POST still reaches the server, the API returns the longer empty-comment message (including after word filters strip everything).
Diagnose and fix
1. Type a comment, then Post.
2. If text vanishes after filters, see Comment cannot be empty / bad-word filter.
3. Confirm comments are enabled for the page.
4. Not a captcha or rate-limit error.