It is recommended against (and thus a restriction placed in our DNS Manager) that you can not create a CNAME record for your root domain. Even apart from conflicts, this wouldn't work.
Now as for the error you are receiving, it's because you already have an A record set for your root domain, and your www subdomain. These are both basic A records created when you make a default DNS zone.
So even if you were to remove the two A records, you still would be unable to create a CNAME for the root domain due to the above limitation I mentioned, but you should be able to create one for www.
However, if you are just trying to make sure your www is used properly, have both use an A record that goes to your IP as they did originally (or do now), and use your virtual host configurations to handle the rest. In Apache you can have your vhost setting pick up example.com as your ServerName, and www.example.com
as your ServerAlias. If you wish to standardize the use of them, you can utilize rewrite rules.