The focus keyword contains a stop word

That’s not necessarily bad for your SEO as Google has become much more apt in dealing with stop words in the last few years. We recommend you check out this post: https://yoast.com/handling-stopwords/

Thank you for the Swift. We will check the article.
On a separate note, Does Yoast (paid version?) handles rel=canonical?

Yes, the Yoast plugin (premium and free) assigns canonical tags to your pages and you can also customize them. You can read more about this here:
https://kb.yoast.com/kb/canonical-urls-in-wordpress-seo/