The main point here is to make sure you have pointed your CNAME record to us (rdtrck2.com) in your domain registrar.
Here you can find how this can be done: Custom Tracking Domain.
After you are all set but still want to check if it returns the correct response, here is a very useful tool: CNAME Lookup.
How to use it
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 rdtrck2.com:
That domain is integrated into RedTrack and can be used for tracking purposes.
If the "Canonical Name" differs or it is stated that DNS record is not found, check the domain settings in your registrar - most likely you have not pointed your CNAME record to rdtrck2.com: