Hello MicheleD,
Thank you for your question. This error usually means the cname record has already been created, you can view/edit all existing cnames in the Advanced DNS Editor.
I would look there first and see if the record exists, if it does edit it. If not contact support, as there may be an issue.
There is also a helpful tool in cPanel for Setting up Google Apps. It really simplifies the steps.
Thank you,
John-Paul