Yoast SEO meta box is blank
If the Yoast SEO meta box looks like this on your posts, pages or custom post types, you know there's something wrong.
Avast is blocking Yoast SEO
The antivirus software 'Avast Antivirus' has a feature called 'Script scanning'. It is supposed to block malicious scripts, but has been reported to block important Yoast SEO files as well.
To prevent this feature from blocking Yoast SEO, you can add your own URL to the 'Script exclusions'. For more information, please refer to Avast's FAQ at avast.com/faq.php.
Outdated Plugins, Themes, and WordPress
Please check and confirm that you have the most recent versions of your plugins (Yoast included), your theme and your WordPress.
None of the above
If none of the above works to resolve the issue please perform a conflict check as described here.