By default, the power pages include a 404 page which something looks like as shown in the below image.
However, this web page is editable. You can find the web page named Page Not Found.
Update the content according to your needs.
If for some reason that web page does not exist, you can create a new one and update the related Site Marker, which is used by the portal internally to show the correct page when 404 status is returned.
You will find a Site Marker named Page Not Found, change the Page value of the Site marker to your custom page. If the Site marker does not exist, you can create one.
Now when someone visits a page in your portal that does not exist, they will be shown your custom 404 page.
