There’s a difference between focus keywords and meta keywords.
You should only have one focus keyword, but can have numerous meta keywords per post.
The Meta keywords field is disabled by default as search engines nowadays don’t really put much stock in it.
Still, if you want to use meta keywords, you can enable it via SEO -> Titles & Meta’s -> General -> check the checkbox for ‘Use meta keywords’ -> Save.
Once you’ve done that you will get the meta keywords template field on the post types tab and you will see the field in the metabox for posts as well.
