If you manually go to /sitemap.xml on your site, do you actually get an XML document?
(it should be pretty obvious)
If not, and you get a 404 page, you may need to visit the ‘Tasks’ section of your dashboard, and then run the ‘Generate Sitemap’ task. That sound then create the actual sitemap.xml on your server.
Thank you. The index is created as xml. Google Console now seems to work with it. It seems confused though because while updating from concrete 5 to concrete 9 i deleted some pages and used different slugs. I wonder How long it takes to forget and delete the history. I hope it will now get an update with the new index.