Prior to a recent update, the WordPress editor had ‘Screen options’ in the top right corner which allows you to set custom fields. However, since the updated editor came along, these settings have moved.
To find them, create or edit a page or post, and click the three vertically-stacked dots next to the cog. Select ‘Preferences’ at the bottom of the page. This is what has essentially replaced ‘Screen options’.
Custom fields should now show up within the editor page (it will prompt you to refresh).