Why were Michael the Archangel and Lucifer arguing over the body of Moses when the old patriarch already was with the Lord?

The story you're referring to is not found in the canonical texts of the Bible but is rather a part of apocryphal literature, specifically the "Assumption of Moses." According to this extra-biblical tradition, Michael the Archangel and Lucifer (also known as Satan) argued over the body of Moses.

The "Assumption of Moses" is an ancient Jewish text that is considered apocryphal or pseudepigraphal, meaning it is not included in the canonical Hebrew Bible or the Christian Old Testament. It presents a narrative about the death of Moses and the subsequent dispute over his body.

In the story, Michael the Archangel is said to have been sent by God to bury Moses, but Satan opposes Michael, claiming authority over Moses' body due to Moses' earlier actions. The exact nature of the dispute varies in different versions of the text, but it often involves accusations against Moses for his role in the incident at the waters of Meribah (Numbers 20:1-13). Eventually, Michael rebukes Satan and claims Moses' body, burying it in an undisclosed location.

The reasons for the inclusion of this story in apocryphal literature are not entirely clear, but it may have been intended to provide an explanation for the absence of Moses' burial place, which is not mentioned in the canonical Hebrew Bible. Additionally, the story reflects themes of divine authority, angelic conflict, and the honor bestowed upon Moses even after his death.

It's important to note that while this story is found in certain ancient texts outside of the Bible, it is not considered part of the authoritative biblical canon by most Jewish and Christian traditions. Therefore, interpretations of this story may vary among different religious communities, and it is not universally accepted as a historical or theological account.