Deuteronomy 4:2 forbids Israel to do what with God's commands?

Answer: Add to or subtract from them

Moses said, "You must not add to or subtract from what I command you," so that Israel would keep the LORD's commandments.

“You must not add to or subtract from what I command you, so that you may keep the commandments of the LORD your God that I am giving you.”— Deuteronomy 4:2 (BSB)