Make sure you have a Video Asset File for the Main Video and optionally an HTML or Image Asset File for the End Card. Also make sure to have Tracker Asset as it is required.
If you have your own VAST Code or VAST URL then End Card and Tracker Asset are ignored
Following are the relevant parameters to use with Create Creative API:
asset_file_id
end_card_id
end_time
name
rewarded_type
start_time
tracker_id
type
: must bevideo