Here are the steps you should follow to fix this issue:
1. Create a text file 'ads.txt'
2. Add the following line to 'ads.txt'
google.com, pub-0000000000000000, DIRECT, f08c47fec0942fa0
Replace 'pub-0000000000000000' with your Adsense publisher-id.
3. Login to your webserver using SSH or any FTP client. Move 'ads.txt' to the root level of your domain. If you are using Apache, the root will be 'public_html' folder.
Within 24 hours, that message should be gone from your account.