I got this message as well.
Error: you have not specified a focus keyword, you'll have to enter one, then save or update this post, before this report works.
This happened after I installed adn activate "Facebook Like - Content Locker".
After deactived, WordPress SEO by Yoast back to normal.
Barukar, I am not sure if your problem due to the same cause, but you can check if this happens only after you install any plugins.
Hope this helps.