I am getting the same issue, however, is the above still applicable? I am still getting the error from the linked topic.
Ps. Why is “Reply as Linked Topic” not available on closed topics? Would totally make sense. 
AI-generated summary
The discussion revolves around the status of header semantic search. MarcP is experiencing an error with semantic search on his instance, despite following guides and tutorials. Falco explains that semantic search requires deployment of a re-reranker model and enabling per post embeddings, and that it won’t work out of the box. He also mentions that it is a hidden experimental setting, but MarcP points out that it was possible to enable it by checking boxes in settings without “unhiding” anything. Falco confirms that semantic search is still an experimental feature, which answers MarcP’s initial question.
Getting an error here?
I mean, where is this happening, here on Meta or in your site?
Ah no, on my own instance. I’m not sure if I ever seen it work over there.
Oh it won’t work out of the box indeed, as it requires deployment of a re-reranker model and enabling per post embeddings.
One of the hurdles with per post embeddings is the storage cost associated with it, and we just made a big advance there via FEATURE: Index embeddings using bit vectors (#824) · discourse/discourse-ai@791fad1 · GitHub.
I know, I’ve followed every tutorial I could find on this. Not using self hosted models, just OpenAI.
That is why it is a hidden experimental setting 
Well that was actually my question (see topic title), thanks.
Confusion arises because there is no hidden setting. As I recall, this should be set in app.yml and below is the only hidden setting that’s not commented out.
tldr; I was able to follow above mentioned guides without “unhiding” anything.. just checking boxes in settings.
I saw a “quick search” here. Is it supposed to work like the instant search experiment? 