Is there a contradiction between making up missed prayers due to masturbation and the principle that "repentance erases what came before it"? And will Allah hold accountable those who die before making them up?

This issue is a matter of scholarly disagreement among the people of knowledge. The more cautious approach, according to the majority of scholars, is to make up the missed fasting days (qada'), contrary to the opinion of Ibn Taymiyyah. There is no contradiction between the necessity of making up the fasts and repentance effacing sin. Repentance removes the sin, but it does not waive rights established as obligations upon one's conscience, based on the Prophet's (peace be upon him) saying: "Allah's debt is more worthy of being fulfilled." Taking the initiative to fulfill this debt is a sign of sincere and complete repentance. Accountability at the time of death depends on the servant's negligence in fulfilling the qada'. If he procrastinates, he is exposed to punishment. However, if he does what is within his capacity before death, then Allah will have mercy on him.