i am 21 and still can't grow a beard.

Beard hair is genetic, look at how it works for your father and your grandfather.
If they are like you, you can think about a beard transplant

No beard is the card you were dealt by your mother and father. It's genetic.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.

Every young man wishes for a full beard that they see on celebs and stars on themselves while they just hit their twenties. However, it is crucial to understand that this wouldn't be possible because the pubertal process begins only during late adolescence.
Testosterone is the androgen hormone found in the male body. It is responsible for the growth of facial hair and other secondary sexual characteristics. As the hormone levels rise in the body, it promotes the vellus hair (peach fuzz) to become coarser and thicker hair over time.
When does facial hair reach its full growth?
Facial hair goes through a lot of modification during the twenties and finally reaches its full growth during the thirties. There are other factors that come into play such as; pubertal age, genetics, testosterone levels etc that determine how fast and how much a patient's beard can grow.
How to spot red flags:
If the patient does not hit puberty at all.
If the patient has hormonal co-morbidities like gynecomastia
If the patient surpasses the rough age of when their male relatives (biological) grew their facial hair.
If the patient does not notice any change for many years.
These are some red flags that may require the assistance of a physician to rule out underlying conditions and treat the patient.
What can be done if one has reached full growth and is dissatisfied?
When patients want a fuller beard or mustache, they can always opt for a beard transplant. This procedure transplants the hair follicles from the scalp to the face to give a much fuller look and feel.

Beard hair growth can mature well into a man's 30's so not having a beard at 21 is very possible. You can look at others in your family also to see if this is a common trait. It is well worth waiting for a few years before deciding whether you wish to do something. At your age your body is natural changing and very easily so could your beard. 

Your facial hair is a genetic inherited pattern. Although some people have had hair transplants on their face, this is often not a good solution.

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

Facial hair loss or thinning is genetic.  It is still taboo soon and you may still grow hair . However if you don't beard transplant can be very successful in the hand s of an experienced Hair transplant doctor.