That would indicate the sitemap isn't present on your filesystem. Have you tried building the sitemap manually to see if you get any errors? By default it's written to the internal_data/sitemaps/ directory.
so the sitemap.php daei is by default in the html/internal_data/sitemaps/ folder. With me it was html/ folder
I've copied them now. Should the sitemap work afterwards and how can I check that?