Can a girl have a very late puberty at age 20?

Not normal. Puberty generally ranges from 9-14 years old. If it is delayed beyond that, there is an hormonal problem that must be evaluated by an endocrinologist.
Late puberty. If this means she has not had a period but has completely developed breasts and pubic hair it is abnormally late and needs an endocrine evaluation. Even more so if she has not fully matured.