I gather you are looking for the keywords box, apparently some smart Yoast employee decided to take it out of version 6.3. They wrongly decided that no one uses it. Wrong wrong wrong, Yoast does half a job in SEO, other SEO tools use keywords.
I have uninstalled Yoast and I am looking for a plugin to put back the keywords field and do a page check. Which is all I used Yoast for.
A classic case of how to make something useful, useless.