What should a girl do who doubts not having fasted Ramadan because she was unaware of having reached puberty at that time?

Certainty is not dispelled by doubt. Therefore, whoever doubts their puberty (being under 15 years old) in a given year, the default assumption is that they have not reached puberty, and they are not required to make up for the fasting of that year. However, for someone who is certain of their puberty but doubts their fasting, and was negligent in fasting Ramadan (definition), they are obligated to make it up. If they were not negligent, then they are not required to make it up, because doubt in this case is a form of obsessive whisperings (waswasah).