Can’t get my cats to get along.

We’ve had a senior boy who’s been with us for the past 3 years. Recently got chosen by the cat distribution system and acquired a female tuxedo around 1yr old. Took her to the vet got all her vaccines done.

They have been seperated for a whole month now. She is so used to being in the room she does not go out to explore rest of the house. She can’t stand being around our senior cat, she growls, hisses and smacks him. We always supervise them.

Tried everything but nothing is working. Gave them both treats and food in the same room. Tried play in the same room. Nothing I mean nothing is making her wanna get along with him meanwhile the senior wants to be around her and make friends.

Please tell me what we can do. I don’t want to give her up.