Final answer:
Aaron couldn't mourn in the presence of God due to breaking God's commandments and speaking against Moses, as stated in Numbers 20:1-13.
Step-by-step explanation:
Aaron couldn't mourn in the presence of God because of a specific event mentioned in the book of Numbers in the Old Testament. According to Numbers 20:1-13, Aaron and his sister Miriam spoke against Moses and questioned his leadership. As a consequence, God punished Miriam by making her a leper for seven days.
It's important to note that this event occurred before Aaron and Moses were allowed to enter the Promised Land. The punishment served as a reminder of the importance of respecting and obeying God's chosen leader.
Therefore, the reason Aaron couldn't mourn in the presence of God was because he had broken God's commandments and spoken against the chosen leader, Moses.