If a property you want to list is already posted for rent on Zillow, the listing will be declined:
Your listing is already being posted from another rental source.
Zillow will only ever publish a single version of a listing at any given time. If the listing is already coming from a listing feed, or another account in Rental Manager, it is very likely for your listing to be declined.
For more information about posting a listing that's for sale and for rent simultaneously, please refer to this article.