It is impossible for a human to grow a tail. While embryos have features that typically resemble tails found in animals, and tails occur in infants in extremely rare circumstances, it is impossible to grow one.
There has never been a reported case of a human growing a tail. In 1984, a case of a tail was, however, found in a 2-week-old infant. This kind of phenomenon is extremely rare and should not be understood as possible or desirable for an adult human. Tails are found in numerous animals, and humans possess them as fetuses while in the womb. Humans have no use for tails as adults.