"Whenever" is the offered correct solution for the exercise "Dès qu'elle porte une robe, elle est belle." Seeing it so often has kind of enforced that one of the translations for "dès que" is "whenever."
Okay, that has been added as well. It may take up to a week for the system to recognize the added translation. In the meanwhile you may still receive an error. If you receive an error after May 5th 2018, please report it, comment, or file a bug report here.