What should a young man do who vowed to study Sharia sciences if he succeeded in the baccalaureate, but after his success, he entered the Faculty of Law (secular sciences), and did not obtain any certificate after 6 years?

If by "Islamic sciences" one means learning the Qur'an and its sciences, Hadith and its sciences, Fiqh, and related matters, then learning what exceeds the individual obligation (fard al-ayn) from these sciences is a communal obligation (fard al-kifaya). Vows related to communal obligations are not binding according to the majority of scholars, but they are binding according to the Shafi'is. Therefore, the mentioned young man is not bound by his vow because it is not binding according to most jurists. His vow is also not binding if he vowed to learn what he is already obligated to learn of individual rulings, because that is already a duty. Man-made laws are not among the Islamic sciences; at most, they are permissible if they do not contradict the law of Allah the Almighty. We advise piety, seeking help from Allah, and diligent pursuit of knowledge.