Custom fields are usually hidden by default. Up until recently users were asked to click on the general posts page and then click on the screen options button in the top right to enable custom fields.
The lack of a custom fields option is usually attributed to plugins which disable the option such as the Advanced Custom Fields plugin. The guideline to resolve the issue for this specific plugin was to add the following code in the functions.php file of your WordPress theme:
However, in WordPress versions 5 and above the option has moved.
On the create or edit page of a post or page users must now click on the three dots (settings) on the top right and from there go to options.
On the pop-up box navigate down and click on options.
In the options pop-up make sure you tick the custom fields option.
Now when creating a post or page the custom fields options are enabled below the main post/page area.
If you want to learn more about custom fields this is good place to start.