That message can only appear when you are writing a reply in one thread, then you navigate to another thread and click Reply. The system asks “where did you mean to post your reply”.
I also see the issue where this reply box doesn’t close correctly after making the reply. If you watch the screen behind the reply box, you can see the post being made, so you can just click Cancel. That is another issue we need to bring up with the Discourse project.
I think we should split this thread starting with post # 9 and close the original thread. It should serve as reference information rather than a discussion. Edit: Done, the original thread was here: Reminder about keeping threads on topic: Reply as linked topic