Why do I have this line behind my hamstrings? (Photo)

The crease is natural, especially over time. Coolsculpting will not work in that area. You could have some minimally invasive skin tightening with radiofrequency like ThermiTight to help soften the area surrounding the crease.

If that has been present your whole life then it is most likely a congenital anomaly most likely a mild form of an amniotic band.
It is not represent a volume depletion so adding volume will likely not correct it and it does not represent excess subcutaneous fat surrounding the area of defect.
Direct excision with undermining may work though scar tissue contracture may form a similar indentation. adding Z-plasty‘s would minimize reformation of the contractile scar tissue but would just add more skin scars.
There’s no easy fix for this one.
You need to ask yourself how much this bothers you and how much you’re really willing to go through for a partial improvement.
Best,
Mats Hagstrom MD