Customize the 404 page with built-in settings, then enhance the template with additional sections. To add sections, confirm you are using a compatible version.
Compatible Theme Versions
Open 404 Template
In the theme editor (Customize), use the page selector to open Others, then 404 page.
Add a section
In the left sidebar of the theme editor (Customize), click (+) Add section.
Select one of the available sections. Use the search field to search by section name.