Botched BBL: my butt now looks deflated and flat.

This type of problem will require waiting another 3-4 months to decide on a treatment. The best material to use to fill this defect will be your own fat. Good luck.

Dear Fisherdoll17,
it would be best to wait until you are fully healed from your first surgery. After that you can discuss your options with your plastic surgeon or ask for a second opinion. Most likely you will need another fat transfer in order to repair the defect. 
Daniel Barrett, MD

Certified, American Board of Plastic Surgery
Member, American Society of Plastic Surgery
Member, American Society of Aesthetic Plastic Surgery

The best thing to do is give your body time to heal. You need to wait 6 months to 1 year after the first surgery to allow tissue to soften down and the final result to come through. At that point the decision can be made if you have enough fat to inject or if sculptra is the best option.