Darius was urged to sign a decree under the law of which peoples, a law that cannot be repealed?

Answer: The Medes and Persians

Daniel's rivals asked the king to sign the document "in accordance with the law of the Medes and Persians, which cannot be repealed."

“Therefore, O king, establish the decree and sign the document so that it cannot be changed—in accordance with the law of the Medes and Persians, which cannot be repealed.””— Daniel 6:8 (BSB)