If you have installed ACF WordPress plugin for creating custom meta boxes, then it removes the the default custom field metaboxes .
The main objective of removing the default WordPress custom field is to increase the speed of load time while editing the posts. Also, you might not need the custom fields if you have ACF plugin installed in your WordPress site. However, if you need to use default WordPress custom field metabox then you can add the following code to your WordPress theme’s functions.php file.
This will solve your custom field Option missing issue.