What's it called when a girl can't have her period? what causes this?

Primary amenorrhea: Lack of menstruation after the age of 15 is called primary amenorrhea. There are several potential causes for this, including anatomical issues with the female organs, hormonal imbalances, malnutrition, chromosomal defects (Turners Syndrome) and more. Your primary care provider can start the evaluation and initiate any treatment available.