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

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.