How can I treat this butt dent? (Photo)

Hi and welcome to our forum!
Several questions...Have you had previous surgery or trauma to the area? I note what appears to be a bruise in the vicinity of the indentation...Is that the result of suction therapy or is the discoloration permanent? 
Suction therapy causes swelling in the area of treatment, camouflaging the indentation. The result is short lived. The fasciablaster is a device which is reported to "loosen fascial adhesions" to treat cellulite. 
If the indentation is of natural origin, lipoinjection into the area may fill out the recess. If the indentation results from previous surgery or trauma, scarring may be responsible for the indentation. This is more difficult to treat, but again lipoinjection will most likely result in improvement of contour. 
You may consider a consultation with a board certified plastic surgeon for examination and recommendations.
Best wishes...

Morpheus 8 is a non-invasive skin and soft tissue tightening treatment that is consistently amazing in our patients. We use the Morpheus to exact fractional induced RF into the skin or tissue in order to tighten collagen and stimulate contraction of the tissue. For acne scars, it is quite useful to build collagen in patients who have atrophic scars but is not very productive for ice-pick scars which require TCA Cross. For tissue tightening, Morpheus is ideal for the face and neck and we often combine it with CO2 laser or Ultherapy to further enhance results. Chemical peels and skin resurfacing should be done at least 5 to 7 days after, but can be performed earlier, depending on treatment severity. Avoid exfoliating skin treatments before Morpheus and expect a period of 2-4 weeks for skin contraction to set in.

Best,
Dr. Karamanoukian
Realself100 Surgeon

These can be improved with a combination of surgery, fillers, and/or lasers and microneedling/prp. I recommend getting a formal evaluation with a scarring expert. you may need cellfina for cellulite, sculptra for collagen production, renuva fat injections or autologous fat injections. Best, Dr. Emer