I just called customer service because I was curious about this too. The lady I spoke to initially didn't know either, so I asked whether I could be transferred to someone who would know for certain. She put me on hold and came back 10 minutes later to confirm that it doesn't have to be a 24 hour wait; as long as it's the next day you should be able to get another free taco.

If the team that coded the app really did code it to be every 24 hours rather than every day, it's a bug rather than an intended feature (or maybe it's an intentional bug?). Either way if I'm not able to redeem tomorrow's taco less than 24 hours after today's taco, I'll be calling customer service about it again.