Following CNAME configuration for your subdomain and after adding your custom domain to your RedTrack account, it's possible that you will be returned with the following response after verifying the domain's DNS settings;
Make sure to wait 24 hours if it is a new domain so that the DNS settings can propagate fully.
Also, if you're setting a CNAME on an older domain, keep an eye out for the TTL (time to live) setting - by default it's 14400 seconds which is 4 hours.
What that means is that your subdomain and CNAME may take that long to propagate.
You can shorten this by modifying the TTL number for your CNAME record.