I just discovered that the root level domain is found through your web host. On bluehost, upload your ads.txt file to your public_html folder.
To verify that you published your file correctly, check that you successfully see your file’s content when you access the ads.txt URL (https://example.com/ads.txt) in your web browser. If you can see the file in your web browser, it’s likely that AdSense will successfully find it.