As Google is giving importance to websites which are mobile-friendly means your website should look good in a smartphone viewing, so it becomes important that your website theme supports mobile-first criterion.
Now Google is sending out notices of “mobile-first indexing issues detected” to web owners via Google Search Console.
What is mobile-first indexing?
In general terms, it means Google will crawl your site from the eyes of a mobile-browser and use that mobile version for indexing and ranking.
What you need to do ?
Make your site mobile-first indexing ready. If you found any issues with your site then correct them.