What triggers this issue?
This issue reports all URLs in the website’s sitemap files that return one of the 4xx (Client error) HTTP status codes.
Why is it important?
Sitemap file of a website must list all the pages you want search engines to crawl and index. Besides, Google uses sitemaps data as one of the factors when selecting canonical URLs.
4xx URLs in sitemaps send a misleading signal to search engines, instructing them to index the URLs that do not exist (404 (Not Found) or 410 (Gone) codes) or the URLs they do not have access to (401 (Unauthorized), 403 (Forbidden), and other). You can check the full list of 4xx HTTP status codes.
This can result in search engines, such as Google, ignoring your sitemaps. In this case, you might end up with some indexability issues on your website.
How to fix it?
Your sitemap files must only include the live URLs that return the 200 (OK) response code.
Remove the 4xx URLs from your sitemaps. If the page is supposed to be crawled and indexed, change its status to 200 (OK).
Note:
The HTTP 429 (Too Many Requests) response codes may indicate that the crawling speed set in the crawl settings for your project is too high for a web server. Reduce it in the crawl settings and run a project re-crawl.