Here're the top reasons why super send might've failed at verifying your custom tracking domain:
#1 You did not wait long enough.
It can take up to a few minutes for the CNAME record you added to propagate (time taken depends on your domain provider).
You can confirm if the record has propagated or not by checking going to a CNAME lookup tool like https://mxtoolbox.com/CNAMELookup.aspx and see if the record has really been propagated yet.
#2 You did not add the right records
Make sure your CNAME record looks like this:
Host name: ss or links or track or anything you'd like to
Value: trail.supersend.io
#3 You're using Hostinger as your domain provider
For some reason, we're seeing issues with validating custom tracking domain hosted by Hostinger. Cloudflare handles this for us and unfortunately, we don't have a solution to it yet. We'll update this doc once we do.