If you’ve modified your site’s robots.txt file to disallow the AdSense crawler from indexing the pages of your site, then we can't serve Google ads on the site.
To update your robots.txt file to grant our crawler access to your site, remove the following two lines of text from your robots.txt file:
User-agent: Mediapartners-Google
Disallow: /
This change allows our crawler to index the content of your site and provide you with Google ads.
Note: Any changes you make to your robots.txt file might not be reflected in our index until our crawlers attempt to visit your site again.