The error on the left it an Apache-style error message. This I cannot explain outright. Possibly the DNS of that computer system or network is stale. It may need flushing/rebooting to clear.
The error on the right is a Netlify error message. This error is showing because the site is built as an SPA (this is likely unnecessary; a purely static HTML/CSS/JS non-SPA site would suffice.) The solution to this is in
Related documentation is Rewrites and proxies | Netlify Docs