Since inception, Google's ranking systems still evaluate the desktop version of a webpage’s content to know its relevance to user's query. But on mobile, the page has less content than desktop. Hence more is the chance that the content's relevancy to user's query is actually less.
There are two chances. Sites can have either desktop version only or both desktop and mobile versions.
Desktop version only -Don't worry. Google will continue to index your site just fine. But remember that the desktop index will not be kept as up-to-date as the mobile one.
Both Desktop & Mobile versions available -Note : If your site is Responsive or Dynamic serving plus the primary content and the markup is different across mobile and desktop, consider serving optimal, content-relevant structured markup.
Use Robots.txt Testing Tool -Verify if your site's mobile version is accessible to the Googlebot .
Use Search Console -Verify your desktop site as well as the mobile version.
Use Structured Data Testing Tool -Compare the equivalence of structured markup across both the versions by typing both the URLs.
Starting to build up a mobile site?
Keep in mind that rather than an incomplete mobile version of the site, a fully functional desktop-oriented site is definitely a better option.