In February of 2018, Yoast SEO dropped support for the meta keywords field in version 6.3. In general, this is because the use of meta tags for keywords has been deprecated by search engines, and is no longer considered a best practice.
They do, however, continue to support meta tags for Twitter, Facebook’s OpenGraph, and other popular meta tags.
More on their site here: https://yoast.com/?s=meta+tags
Hope this helps!