hard to say without the URL, just post it
i have seen everything, from page set to noindex, from page blocking googlebot, from dns resolving errors, from special http headers just for googlebot, from google not being able to see anything on the hompage, from hidden spam issues, hacked sites, from robots.txt disallowing /, from x-robots-tag noindex at cdn, from rel=canonical to the wrong domain, from 302 chains and soft-404 homepages, from mixed http/https splitting signals, from geo redirects trapping bots, from bot protection blocking googlebot, from server 5xx timeouts under crawl, from spa content with empty html, from blocked css/js so rendering fails, from cloaking-by-accident via ab tests, from staging/dev subdomains indexed, from domain move with missing 301s, redirecting away from the domain root, from self linkspam due to "branded backlinks" ...
run the 3 SEO tests https://www.fullstackoptimization.com/a/seo-basics#:~:text=t... on the root domain which pretty much identify 80% of all onpage (but not onsite) issues and report back