Please follow the below step to add it.
- Click on the "Fix now"
- Click on the "Download" and download your "ads.txt" file and open it.
- Copy the command
- Go to the "Settings" in your blogger site.
- Find the "Monetization"
- Enable the "Custom ads.txt"
- Click on "Custom ads.txt"
- Paste the command which copy from the downloaded "ads.txt" file.
- Click on "Save".
You can ensure it correctly working or not as the follow.
- Add /ads.txt after your site URL.