There is a bug in regards to sitemaps which presumably is fixed in 2.2.*, that they are placed in Magento root directory after creating them from Marketing -> Site Map Therefore they are not publicly accessible via browser or by crawlers.

To solve this we can set up a symlink in pub directory By typing command below

cd {webroot}/pub
ln -s ../sitemap.xml

Now after you visit sitemap is accessible.

Bug issue link