First you need to edit a post or page where you want to add keywords and meta description. Scroll down a little to ‘Yoast SEO’ meta box below the post editor. You will see your site’s title and automatically generated description as a snippet. Below that you will see Meta Keywords field.
For More Information Please Refer: https://www.wpbeginner.com/beginners-guide/how-to-add-keywords-and-meta-descriptions-in-wordpress/