Once you've run a crawl of your site in the Site Audit tool, you might see a '4XX page' error in the report under Internal pages menu:
This error indicates that the requested page or resource cannot be accessed due to a bad or broken link.
Most common 4XX Client Errors:
401 — Unauthorised
403 — Forbidden
408 — Request Timeout
404 — Not Found
Why is it important for SEO?
Pages with 4XX client error can hurt your website crawlability, because search engine bots will be coming across links to dead or broken pages from your website. This can damage your visitors' browsing experience, when they click on a link and see "Oops, page not found" error message.
It is important to remove the bad or broken links or replace them with relevant links to live pages.