Hello,
> The line
>
> (sitemap-filename (concat root (or sitemap-filename "sitemap.org")))
>
> in ox-publish.el prevents the user from using an absolute path for the
> location of the sitemap. This is also not very good practice, because
> concatenation does not guarantee good pathnames.
Remember to cover the basics, that is, what you expected to happen and
what in fact did happen. You don't know how to make a good report? See
https://orgmode.org/manual/Feedback.html#Feedback
Your bug report will be posted to the Org mailing list.