CNAME: Conflicts with pre existing RRset

You can get this under two conditions:

1. You’re trying to add a CNAME record for your-domain-name.com
2. You’re trying to add a CNAME record for something.your-domain-name.com

In the first case, you absolutely can’t add a CNAME because for your basic domain they are required records and CNAME is incompatible with any and all other records including that required records.

In the second case, if you really want to use CNAME, you’re going to have to look at all the other records for something.your-domain-name.com and ensure that you either don’t care about them or that they exist in the other domain to which you’re trying to point something.your-domain-name.com. Once you’ve decided you really don’t care about any of those records, you’ll delete all of those something.your-domain-name.com records and then add your CNAME.

Technically when a program encounters a CNAME, it treats that domain as if it were the domain referenced by the CNAME, and thus your domain cannot have any other records because those would collide with the records from the target domain from which it’s been told to get all records.