I'm 20 years old still i don't have facial hair on my cheeks, why is this?my dad has a full beard.

Maybe your mother gave you the genes for no facial hair.Facial hair is a genetic trait that you inherited from your  mother or father. It is not generally related to hormones or a disease process. Some may try to use Rogaine but without great success.You may want to consult with a doctor for possible surgical options.

Many young men want to accelerate their beard growth and ask
about it. It takes age (genetic triggers which occur in family lines) so
if you want to find out when your beard will grow out, you need to find out
when your father or grandfather started to grow theirbeard. Hormones should have kicked in
nicely by the time a man reaches 18 but he must wait out his genetics
as hormone alone are not responsible for the onset of beard grown. Beard
transplants can be done when the young man is a bit older (over 25) if the
beard has not grown in yet

Thin Facial hair is genetic, however at this age it is still too early and you may still grow some hair.  If you don't then beard transplant in the hands of a experience hair transplant surgeon is very successful and looks natural.