Don't worry, it's not a serious warning. You need to put one text file in the root of your domain. This text file knows as ads.txt (Authorized Digital Sellers).
- Download ads.txt from Google AdSense
- Upload ads.txt to your root of the domain using cPanel ➔ File Manager or FTP
If you are not sure about the root of a domain. Then it's the path where your index.html or index.php is.
How long does it take Google to verify ads.txt?
In most case, Google verifies ads.txt within 24 hours. Verification time highly correlated with website request rate. If your site making few requests then Google AdSense can take up to few days to approve your request.