I read a topic that for those who have already caught Darkrai, they need to defeat Horon without losing any pokes to be able to enter the room to do the Gengarite mission, I defeated Horon without losing any pokemon and I still can't enter the mission room.

Thank you very much in advance

Hey, that is not fully correct.

If you cant enter the Chamber for the quests it is usually because you did not beat Umbra in the Dark Realm during the Darkrai quest as that part was optional.

You would need re-do the Dark Realm quest and beat Umbra this time, make sure to recruit all pokemon as well because you will need them for the following quest/s.

To go back to the Dark Realm you already did correct by beating Horon without letting a pokemon of you faint, now you should able to re-enter Alucards castle where you have to beat him again while having a pokemon with the move Miracle Eye in your party. Doing that will send you back to the Dark Realm.

thank you for the explanation

now it worked

Locked as resolved. 

Please don't be afraid to start a new thread if you have any problems or questions in the future.