how index subfolders?

If I search this page:

https://lnx.sinapsi.org/public/triangoli.htm using:

https://search.google.com/search-console?resource_id=sc-domain%3Asinapsi.org

I get: the URL is not indexed. So how could I index the subfolder https://lnx.sinapsi.org/public/ ?

My sitemap is located here: https://www.sinapsi.org/wordpress/sitemap_index.xml

But how could add a site map here:

https://lnx.sinapsi.org/public/sitemap_index.xml

Thanks.

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