There are several reasons for that
1) Sitemaps is not in the correct format. It should be example.com/sitemap.xml
2) DNS records problem
3) Redirection issue
4) Google bot May be busy
- First you should try to submit the sitemap to inspect url , check is it discovered by Google or not. If it is not discovered than you should check for test URL and if it showing these error Failed: DNS error: Host unknown. You have to wait for 24 hours.
- The best way to add a sitemap is you have to add the domain name with https in property. If you add a naked domain ,actually it would redirect to the https first and then dns. And DNS verification may not be occur due to security reasons. So better to add your domain in URL prefix session as https://knowledgeassam.com instead of knowledgeassam.com in domain section. See the above picture.
- Check wheather your sitemap is valid or not, Click here
What is sitemap?
Sitemap is file where the metadata or information about the pages and post of the website is available. Sitemap is blueprint for a website. When Google crawl the website it's used to find details of the website in the sitemap. It helps search engine to find your website and indexed your post in the search engines. Eg knowledge Assam has a website called knowledgeassam.com and it's sitemap would be knowledgeassam.com/sitemap.xml and knowledgeassam.com/sitemap-pages.xml . Using these sitemap Google retrieve the information of the website.
Why is it important?
It's important because without sitemap it will not rank your website in Google. Moreover to get organic views and SEO friendly website you have to make sitemap.