im done quest darkrai and other quest but why this gengarite quest i canot join please help or any guide for this?

You have to beat the Mega-Gengar in the dark realm to enter that room.

To go back to the dark realm to do so you have to respawn Alucard in his Castle on the third floor, by defeating the Horon boss in the Dark Valley (north of Necropolis).
Make sure to not let any pokemon of you faint while battling Horon. If you didnt get the Rapidash Mount from Horon yet, you will get rewarded with the mount on beating Horon. You have to wait 3 days for Horon to respawn after that. If you beat him again, Alucard will respawn.
Defeat Alucard with a pokemon that has the move Miracle Eye on your party and you will find yourself on Dark Realm again.

Also you should recruit all the pokemon in the dark realm if you didnt do that on the first time being in the dark realm, as you will need those for the upcoming quests.

4 hours ago, Norex said:

You have to beat the Mega-Gengar in the dark realm to enter that room.

To go back to the dark realm to do so you have to respawn Alucard in his Castle on the third floor, by defeating the Horon boss in the Dark Valley (north of Necropolis).
Make sure to not let any pokemon of you faint while battling Horon. If you didnt get the Rapidash Mount from Horon yet, you will get rewarded with the mount on beating Horon. You have to wait 3 days for Horon to respawn after that. If you beat him again, Alucard will respawn.
Defeat Alucard with a pokemon that has the move Miracle Eye on your party and you will find yourself on Dark Realm again.

Also you should recruit all the pokemon in the dark realm if you didnt do that on the first time being in the dark realm, as you will need those for the upcoming quests.

Alucard not spawn

Are you sure you beat Horon without letting one of your pokemon faint and the Rapidash didnt spawn for you to get the mount?

3 minutes ago, Norex said:

Are you sure you beat Horon without letting one of your pokemon faint and the Rapidash didnt spawn for you to get the mount?

Yes my poke not fain and i got mount too but alucard not spawn

If the Rapidash spawned for the mount, you have to battle Horon again in 3 days to respawn Alucard, like my first message stated.

8 hours ago, Norex said:

If the Rapidash spawned for the mount, you have to battle Horon again in 3 days to respawn Alucard, like my first message stated.

Ok thanks i try