The Custom Fields option was missing from the WordPress screen options. After some investigation, we were able to find the cause of the issue.
If your WordPress site is missing the custom fields option under the ‘Screen Options’ menu, then you need to check if you have the Advanced Custom Fields (ACF) plugin active on your site.
ACF is a very popular WordPress plugin that developers use to create custom meta boxes. ACF is running on over 1 million websites.
In ACF version 5.5.13, they added a setting to remove the default WordPress custom field metabox to speed up the load times on the post edit page. The idea being that you shouldn’t need the default metabox since you’re using ACF.
However if you need to have the default WordPress custom field metabox enabled, then you need to add the following code to your WordPress theme’s functions.php file.
This will solve the custom fields option missing issue in WordPress.