Google search console fails to fetch sitemaps | "Sitemap could not be read"

This is a known bug. See this Google support answer.

Try using the URL Inspection on the sitemap url. it SHOULD say not indexed (it's a sitemap not a page, so shouldn't be indexed!), but can use the Live Test, to check if Googlebot CAN fetch it! If Google can fetch it, it's most likely just Pending, which case just wait.