How to reproduce
Have an old Hugo site on the default *.netlify.com subdomain
By old I mean that it was first configured before the switch to the *.netlify.app subdomains.
In the sitemap XML all specified URIs still use the *.netlify.com subdomain.
Impact: For your *.netlify.app site you cannot register the sitemap in Google’s webmaster tools as it rejects the sitemap saying that it can only process URIs on the same subdomain (i.e. with *.netlify.app).
netlify.toml in your Hugo project and edit
[build] command = "hugo --gc --minify -b $URL" publish = "public"
[build] command = "hugo --gc --minify -b https://example.netlify.app" publish = "public"