You can verify the status of your ads.txt file in the Inventory > Sites section.
If the ads.txt file is either absent or incomplete due to missing lines, an alert message will be displayed next to the respective site:
By selecting the "Update your ads.txt" button, you can download the appropriate version of the ads.txt for your site.
Keep in mind that the ads.txt varies for each site, so ensure that you insert the correct ads.txt file.