The 24 hour period is the hint. This is common with late bookings, low availability bookings and bulk tickets. There is some possibility that Delta will not accept the booking and they have 24 hours in which to do so. Probability of not being ticketed is usually low, but still finite.

Wait 24 hours and see if your itinerary shows up on delta.com. If there, you are ticketed. If not, start looking for alternatives.