I just had the same problem. This was my site: peterplevko.eu/sitemap.xml - but it said 'Could not fetch.' I had to add a slash / at the end of the URL, so it became peterplevko.eu/sitemap.xml/ , and suddenly it started working. The solution to this problem was mentioned in this thread: github.com/vercel/next.js/issues/75836.

As seen here: