The main point here is to make sure you have pointed the correct CNAME record to us (your dedicated tracking domain) in your domain registrar.
1. Go to https://mxtoolbox.com/CNAMELookup.aspx, put your domain name to the box and press "CNAME Lookup" button;
2. Check Canonical Name - it should be your dedicated tracking domain.
If the "Canonical Name" differs or it is stated that DNS record is not found, check the domain settings in your registrar, as most likely you have not pointed your CNAME record to your RedTrack dedicated tracking domain. To do that check out the article Adding a custom domain.