I'm 20 and my moustache is not growing in the middle part. (Photo)

Facial hair growth is a genetic trait.  If you have very little facial hair there isn't much you can do to increase the growth (other than a hair transplant surgery).

A mature beard and mustache takes hormones and time. You should wait until you are 26 before considering any treatment that is surgical

A hair transplant in this area is quite common as well as other parts of the beard area that people just cannot grow hairs. If you are wanting to add hairs to this area, seek the advice of a specialist that has good reviews and that you trust.