It is no longer possible to create app download cards. Trying to do so will result in the following error:
$ twurl -X POST -H ads-api.twitter.com "/1/accounts/18ce54d4x5t/cards/app_download?name=deprecated&app_country_code=US&googleplay_app_id=com.twitter.android" | jq
"message": "Since 2017-03-29, it is no longer possible to create app download cards. Instead, we encourage the use of image and video app download cards."
As a reminder, it is still possible to create image and video app download cards.