you did not mention your height, by which BMI can be calculated accurately.
It's ok, but 30 kgs is very less for 20 year girl. It can be due to hormonal
imbalance, chromosomal disorder, etc., Using step up powder without knowing the cause , can lead to furthur complications.
Better know the actual cause of low height and weight and then try to treat it. Consult an endocrinologist
for complete assessment.