On the site Settings, all items are checked of Search Engine as attached, but only robots.txt is created for search engines. What needs to be done to create a sitemap.xml file along with robots.txt?

On the site Settings, all items are checked of Search Engine as attached, but only robots.txt is created for search engines. What needs to be done to create a sitemap.xml file along with robots.txt?
Hi.
I suppose it takes more than 1 page (index) in the project to generate a sitemap. But i’m not 100 % sure. But it would make sense.
Mr. F.