In December 2016, we made the following announcement, letting users know that creating app download cards would no longer be supported:
Today, we’re announcing the next step in the deprecation process. As early as 2017-06-08, we will remove the ability to edit existing app download cards. This only affects the PUT accounts/:account_id/cards/app_download/:card_id endpoint.
It will still be possible to view and use existing app download cards—as well as all image and video app download cards—in mobile app promotion campaigns.
Twitter Ads API Team