If someone visits a link that is no longer valid for your site the browser will show a 404 Not Found error. It's helpful to customize that error page so that you can point the visitor to try another page of your site like your homepage.

There are two types of 404 pages: one for your main site design and one for your blog. 

To create a 404 page for site that does not have a blog, simply add a new blank page and name it exactly "404" as shown in the screenshot below. When published it will automatically be used as the 404 error page if someone goes to a link that's not valid for your site. You can then customize this page with a custom 404 message and navigation to your homepage.

If you have a blog tier Showit subscription, you will want to create a custom 404 page under your Blog Templates. Choose the + icon to the left of the Blog Templates in the left side Site Panel, and choose to Add blank Blog Template.

Once you have renamed your WordPress 404 page, you will then need to set the WordPress Template for this page. You can do so by clicking on the name of the WordPress 404 page, then navigating to the right side Properties Panel, and under the Template info tab set the WordPress Template to 404

You can then customize this WordPress 404 page, adding a custom message, background, and navigation that points you back to your homepage.