If you are having this problem and you are a blogspot / blogger user then it's not a big problem for you. It's very very easy to fix this, you need to follow the step by step guide below.
1. Click on the Download (you will find the link just few lines below the warning) link and download the ads.txt file.
2. Click on the Settings (Gear icon).
3. Click on the Search preferences.
4. On the Custom ads.txt line click Edit and chose Yes and then go to you downloded ads.txt file, open it, copy the content, and come back and paste the content in the box, and then click Save changes. That's it. You're done. I hope this helps you.