Albert Barnes Commentary Numbers 23:21

"He hath not beheld iniquity in Jacob; Neither hath he seen perverseness in Israel: Jehovah his God is with him, And the shout of a king is among them." — Numbers 23:21 (ASV)

“Iniquity” and “perverseness” are found together again in the Hebrew of Psalms 10:7; Psalms 90:10, and elsewhere; and signify wickedness together with that tribulation which is its proper result.

The shout — The word is used (Leviticus 23:24 note) to describe the sound of the silver trumpets. The “shout of a king” therefore refers to the jubilant sounds by which the presence of the Lord as their King among them was celebrated by Israel.