If you are using Block Editor, to make the custom field option visible, click the 3 dots on the top right page, click Options, checked “Custom fields”. If you are unable to find your “Options” menu, it means the computer screen is too short so the drop-down menu is not visible fully. To fix this, use Classic editor as explained in the next part.
If you are using Classic Editor, click “Screen Options” to enable the Custom Field box. Make sure the Custom fields box is checked.