Will God forgive the sin of one who made a covenant with God not to commit fornication, then committed fornication, and what must he do to atone for his sin?

A vow to abandon something forbidden is not considered binding by the majority of scholars. However, it is more cautious to offer an expiation for an oath (kaffarat yamin), which is to feed ten poor persons, or clothe them, or free a slave. If one cannot find any of these, then to fast for three days, based on the Prophet's (peace be upon him) saying: "The expiation for a vow is the expiation for an oath." As for the forgiveness of sins, whoever has sinned should hasten to repent, for Allah the Almighty accepts a servant's repentance as long as they have not reached the rattling of death, and He replaces the evil deeds of the repentant with good deeds if they are sincere, as stated in His (the Almighty's) saying: "Except for those who repent, believe and do righteous deeds. For them Allah will change their evil deeds into good ones. And Allah is ever Forgiving, Most Merciful."