Are you 100% certain that you finished the Halloween quest? Meaning that you had that final talk with all 3 mayors in Murky Town? Also beating Kaguya/Zetsu might be part of that requirement, I don't have any confirmation on that yet.
A guide on how to get Darkrai during the Halloween event of 2017
- At least 100h played
- Finish part 1 of the event from what I was told by players so far, this includes beating Kaguya but not Zetsu
- Finish main story (Until Sinnoh)
- Only catchable during 0:00AM to 2:00AM...