@majoritasdev is correct. Scheduled Tweets were released as part of the Ads API, which means you need to apply for access if you wish to access these endpoints. While it’s possible to scheduled Tweets outside of the Ads API—with TweetDeck, as you mentioned, for example—there aren’t endpoints available outside of ads, to my knowledge.
The Tweet endpoints (regular or scheduled) in the Ads API add the ability to nullcast Tweets (these do not appear in the public timeline and are not served to followers) or to Tweet as another user (as long as the correct permissions have been granted).
Hope this helps, @jtestk.