ads.txt is missing or out-of-date warning....

Hi there, 

I know there have been some posts about this in the past, but I think this might be a new problem. 

I'm getting the alert…

Your ads.txt is missing or out-of-date. This will affect your earnings if not corrected. If using Grow for WP, troubleshoot here. If not, follow the manual instructions below and then refresh this alert.

And I've followed the instructions, refreshed, but it's still there. 

From earlier conversations about this it seemed there was a new version of ads.txt to use, but I've used the one that's being suggested so I'm not sure how to fix it.

Can you help? 

Thank you!