If someone visiting your site happens upon a page that does not exist, they’ll automatically see a Weebly 404 Error Page like this one:
The error is very straightforward and essentially says, “The page you’re trying to visit does not exist, but you can click here to go to the site’s home.” It works.
But what if you want your own 404 page instead of relying on Weebly’s default error message? You can easily do so.
Go to the Pages section of the Editor and use the “+” button to add a new Standard Page. Name that page “404” and check the box to Hide in Navigation.
Save this page and you can have it say whatever you want, show whatever you want, look however you want. It works like any other page, except it’ll only show up for visitors who somehow end up visiting a page on your site that doesn’t actually exist.