Does sin befall the questioner for making monthly payments on his father's usurious loan, in exchange for receiving the principal amount of his saved certificate?

Borrowing with interest from banks is forbidden because it is usury (riba), and the investment certificates issued by banks in your country are also forbidden.

You are obligated to repent from engaging in forbidden investment certificates and to get rid of them by taking the principal and spending the usurious interest on the welfare of Muslims. Your father must also repent from borrowing with usury.

There is no sin upon you for repaying your father's forbidden loan, because its principal and interest must be repaid if it is not possible to avoid them. There is no sin in helping your father get rid of his debt.

However, the cautioned matter is the continued existence of the usurious certificate. If you can cancel it, you must do so, even if it leads to not repaying your father's debt, which is not religiously obligatory upon you. But if you cannot cancel it because it is mortgaged against the loan, then there is no sin upon you in continuing to repay your father's debt.