Google discovers your site by crawling: following links and reading two small files that describe what your site contains. When they are missing or wrong, Google may be slower to find and index your pages, and you would never receive an error message about it.
robots.txt tells crawlers what they may visit and where your sitemap lives. The sitemap is a machine-readable list of every page you want found. Together they give Google a complete list of your pages instead of leaving each one to be found by following links.
Some sites have no sitemap. Some have a robots file that quietly blocks pages it shouldn't, a leftover from a redesign is the classic case. Either way the symptom is the same: pages that exist but don't show up, or take weeks longer than they should.
Generating a correct sitemap, referencing it from a sane robots file, and checking nothing important is blocked. Our free review checks both files; the $40 tune-up corrects them where your platform allows.