What if I leave Yoast focus keyword empty?

All the Focus Keyword in Yoast SEO does is help you guide your choices in the other SEO options for the post, it doesn’t actually add anything to the post itself.

For example, if your post is about sandwiches, you’ll enter sandwiches as your focus keyword, and Yoast SEO’s checks will make sure that your post content, title, description, and such all mention sandwiches.

thank you James ??

Actually I know that about focus keyword, but sometimes I can’t choose the perfect one for my post, and I wonder if I can leave it empty so google will decide when to show it in search results depending on the content and the title.
I don’t want to add focus keyword that harm my post SEO, so I think it’s maybe better to leave it without one ?

The focus keyword won’t show in your results or on your site, that’s what I was trying to say above. ??

It’s only used by the Yoast SEO plugin to guide your choices that *are* seen by search engines, it’s only ever seen by you in the editor, never by anything else anywhere else.

thanks again ??

I know it won’t show, I was wondering if I published my post without adding focus keyword -in Yoast plugin-, will that harm my post appearance in google search results ?

No, because Google never sees the focus keyword, no one but you ever sees the focus keyword.