To get this opportunity, you need to fulfill three conditions:
1. Complete the Manor and defeat Edith (as far as I understand, you did this).
2. Complete the repair of the side room at the Milk Farm. You can order the repair in New Ark at the Hardware US store. The cost of the work is 10,000 credits. After ordering, some time must pass. You will have to look at the farm from time to time and check if the repair is finished.
3. Rescue the cowgirl kidnapped by Pixie. To do this, go to Pixie's lair (the place where you first met her). The cowgirl will be in a cage in the lower left corner. You need to talk to Pixie and feed her pets - then the cowgirl will leave the cage and go to the farm.
You should also take into account that during the repair, Bell may disappear from the farm. In this case, you will have to find her and return her. The first time she can be found in the Forest Park, the second - in Transylvania, the third - in the police station in New Ark. To prevent these disappearances from happening again, you can buy a cowbell from the Naughty Kitty store in New Ark and give it to Belle.