The Google ad AdSense crawler visits your site to determine its content in order to provide relevant ads. The crawl is performed automatically and Google is not able to accommodate requests for more frequent crawling.
If the URL sent to Google points to a page that doesn’t exist (or no longer exists) on a site, or results in a 404 (Not Found) error, Google’s crawlers will not successfully crawl your content. To ensure Google can crawl your sites make sure you’ve given access to Google’s crawlers. This means enabling Google’s crawlers in your robots.txt.