You cannot comment on this folder
Same ACL idea as file comments: the folder is not viewable on the public folder page for this visitor (private / not owner).
Why this happens
assertFolderViewableOnPublicPage returns 403 before create. Public folder comments must also be enabled for posting to work at all.
Diagnose and fix
1. Owner: make the folder public / shareable as designed.
2. Visitor: use a public folder link while signed in if required.
3. Admin: Public folder comments on; Enable file comments / related toggles as needed.
4. Distinct from reply-depth rules and rate limits.