Sitemap is not declared in robots.txt
What this means
A sitemap exists but robots.txt does not point to it. Search engines that have not been told where the file lives have to guess at conventional paths, and crawlers other than Google frequently do not guess.
How to fix it
Add a Sitemap: line to robots.txt with the absolute URL of the sitemap or the sitemap index.
Fix effort
This check needs a developer fix: it turns on a routing, hosting or infrastructure decision that no automatic edit can make safely.
Authoritative source
https://developers.google.com/search/docs/crawling-indexing/robots/create-robots-txt
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