google sitemap (Sitemap could not be read / Couldn't fetch)

Hey
why google sitemap (Sitemap could not be read / Couldn't fetch)?
My site sitemap couldn't fetch
Robot.txt is default settings..
Sitmap.xml :
couldn't fetch but
Bute this
Sitemap:
atom.xml?redirect=false&start-index=1&max-results=500 is successful ..

You have got to validate your xml sitemap to ensure it's well informed and constructed correctly .
Enter the xml URL on google and check if you can access it from there.
The validator would tell you that there is something wrong the with the admin .xml file

First know if it is a static sitemap that you created and uploaded or it is a dynamic sitemap generated with plugin or functionality on the website platform. In static sitemaps, which are created manually, the chances of syntax errors are more so you need to fix that first. Can use any validator or can just check in your browser and see if it gives any error.
If it is a dynamic sitemap then you might need to check with the plugin support or it could also be a hosting issue due to which Googlebot is not able to crawl the sitemap page.