If you not able to see custom fields option in screen options, you will be worrisome if you want to change something from WordPress posts backend. The root cause this be Custom fields plugins. This plugin is commonly used to add more fields in WordPress backend. This gives you more options to add in the backend of WordPress website.
How to Fix Custom Fields Not Showing in WordPress?
If you have Advanced Custom field installed on your WordPress website, then you would not be able to see this option. To enable default custom fields option in WordPress backend you would need to add this line of code in functions.php file. Then you can see this option in WordPress beackend.
add_filter('acf/settings/remove_wp_meta_box', '__return_false');
Pasting this code will enable, default custom fields option in screen options.
Results after you added this code:
