The person registered for
DAS needs to have entered the park before you can schedule a DAS Return Time. Anyone in your group that you want to add to a DAS Return Time also needs to have entered the park.
Not all attractions are open right at park opening. You can only add a DAS Return Time for attractions that are open AND have a posted Standby Wait time.
Also, if an attraction closes, you won't be able to make a DAS Return Time until it reopens