The reason given is that Israel “saw no form” when the LORD spoke at Horeb, so they must not act corruptly by making an image Deuteronomy 4:15–16. The passage warns against turning to worship created things, including the sun, moon, and stars Deuteronomy 4:19. It also describes the LORD as “a consuming fire” and “a jealous God” in this context Deuteronomy 4:24.
Moses warns Israel to watch themselves closely because, at Horeb, they heard Yahweh speaking but saw no visible shape Deuteronomy 4:15. Therefore they must not make any carved image—human, animal, bird, creeping thing, or fish—or be pulled into worshiping the sun, moon, and stars. This unit sits inside Moses’ long address that combines recollection and warning as Israel prepares to enter the land. Just before this, Moses highlights the uniqueness of Israel’s experience: hearing God’s voice and being brought out of Egypt, then urges careful obedience for life and stability Deuteronomy 4:9.
Deuteronomy 4:15–24 ties Israel’s worship directly to what they experienced at Horeb: they heard Yahweh speaking “out of the midst of the fire,” but they “saw no manner of form” (explicit). Because there was no visible shape, any attempt to represent Yahweh by a carved likeness is treated as self-corruption (explicit).
A careful answer keeps the immediate passage, the book's argument, and the whole-Bible storyline together, so the question is answered in context instead of as a detached idea.
Start with Deuteronomy 4:15-16, then read the surrounding section so the answer stays connected to the passage's flow.
15Take therefore good heed to yourselves; for you saw no manner of form on the day that Yahweh spoke to you in Horeb out of the midst of the fire.
16Lest you corrupt yourselves, and make yourself an engraved image in the form of any figure, the likeness of male or female,