Go to your dashboard > indexing > sitemaps > add a new sitemap
Here you see Google Search Console requires a URL, but with Google Sites you cannot upload or attach an XML sitemap directly. We have no access to the hosted files or directories. Therefore, as it stands, this is impossible, but the situation is not hopeless. In fact, sitemaps are a limited part of the whole SEO equation.
On your Google Site, in editing mode, use the three-dot menu in the top right and select "Help Sites Improve" to submit written and screenshot feedback. Google will consider this feedback as a contribution to request top features to possible include in future patches / updates to the platform.
This is quite the subject and we have compiled resources to assist your SEO / SEM endeavor. Therefore, for help check out the Google Sites Resources page. We anticipate creating a full fledged page for this content soon.