Whenever you login into Google Adsense you may see something on top of your Google Adsense in Red background color which states that
Ads.txt is missing in some of your websites and your earnings are at Risk!
What is actually Ads.txt?
From Google’s explanation
Authorized Digital Sellers, or ads.txt is an IAB initiative that helps ensure that your digital ad inventory is only sold through sellers (such as AdSense) who you’ve identified as authorized. Creating your own ads.txt file gives you more control over who’s allowed to sell ads on your site and helps prevent counterfeit inventory from being presented to advertisers.
In simple words it’s a mechanism which allows you the control the provides who can sell ads on your site! It’s more useful for advertisers rathernthan publishers..
To fix this on your site
Just add a ads.txt at root folder of your site something like “www.site.com/ads.txt”
with something like this show below