Support for the Authorized Digital Seller (ads.txt) protocol has been launched for / blogs. So, you can now manually set up the content of the ads.txt file via Blogger settings without the need of uploading any file to Blogger’s root directory, which doesn’t exists.
If your blog is not monetized, or it’s only configured to use AdSense using the Blogger-AdSense integration, then you do not need to manually set up ads.txt. Blogger will do this for you.
If your blog is monetized with third-party providers or you’ve manually integrated AdSense in your Blogger blog, you will need to manually set up the content of the ads.txt file.

How To Set Up an ads.txt File On Blogger Manually

-Sign in to Blogger at blogger.com
-In the top left, click the Down arrow Down Arrow .
-Click the blog you want to set up an ads.txt file on.
-On the left, click Settings.
-Under “Settings,” click Search preferences.
-Under “Monetization,” find “Custom ads.txt” and click Edit.
-Click Yes.
-Copy the settings from your third-party monetization provider and paste them in the text box.
-Click Save settings.
-Blogger Ads.txt File Set Up for Adsense
-If you want to set you your Blogger ads.txt file to publicly declare that Google is authorized to sell your ad inventory, copy and paste the code below in the ads.txt box :

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for more info. About this tutorial.

For a comprehensive post on How to setup ads.txt file for both WordPress and Blogger platform, see
https://www.techsblog.com.ng/2019/06/05/how-to-fix-ads-txt-error-on-google-adsense/

Thanks for the wonderful guide. But one thing most guides missed out is to check DNS records whether they are correctly configured. If not, you can never dismiss the message. It's in fact the first and most important step. Also here's a quick guide on how to get adsense approval for a new blog: https://iazd.com/how-to-get-adsense-approval-for-a-new-blog/