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:
What triggers this issue?
This issue reports all internal page URLs that returned one of the 4xx (Client error) HTTP responses code to our crawler.
Most common 4xx Client Errors are:
401 — Unauthorized
403 — Forbidden
408 — Request Timeout
404 — Not Found
429 — Too Many Requests
Why is it important for SEO?
4XX pages 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.