I want to ask a question. Last year, when I was on the darkrai quest, Halloween's island was closed. I want to carry out the quset again. Who do I need to visit? Whenever I try to enter the castle, I will be rejected. now i dont have the darkrai

If you are unable to enter the castle you have to beat the Horon boss first. You can find him in the Dark Valley which is located North of Necropolis. If you are able to beat that boss without one of your pokemon fainting you can enter the Castle again where you can find Alucard. You now have to battle Alucard while having a pokemon with the move Miracle Eye on your party and after that you will find yourself in the Dark Realm again.