That message usually comes up when you've misspelled the genus name. Just enter Phlox as the genus and Creme de la Creme as the cultivar. I would create the entry for you, but it's best to have Phlox proposals approved by the moderator of the Phloxes database, @KentPfeiffer.
Name: Suzanne/Sue Sebastopol, CA (Zone 9a) Sunset Zone 15
A tip: check current entries to see how they are listed. You can even click the "edit plant name' to see exactly how and where to list the , common, genus and cultivar names. Just back out once are done looking at it.