Meta keywords template in Post Types
Use %%tag%%, %%category%% for Meta keywords template in order for the post's tags and categories to be used as meta keywords. It will generate something like e.g.:
<meta name="keywords" content="SEO, Yoast, How To"/>

See also in the Help center the Basic Variables and Advanced Variables. 
https://kb.yoast.com/kb/yoast-wordpress-seo-titles-metas-template-variables/