Is it permissible to study Sharia without the father's knowledge, and what do you advise a girl who wishes to study Sharia but whose father refuses?

Your abandoning postgraduate studies is not considered an act of disobedience (ʿuqūq) towards your father, for a father does not have the right to compel his child in personal matters such as study. It is advisable that you explain to your father your desire to specialize in Islamic sciences, and that you tried to please him but were unable, and that you are keen to please him as much as possible. If you can combine both studies without compromising religious obligations, that would be better. If the father insists that you abandon seeking knowledge and engage in commerce, you are not obliged to obey him in that, and this is not disobedience. Formal study can address confusion in seeking knowledge.