John Gill Commentary Deuteronomy 17:15

"thou shalt surely set him king over thee, whom Jehovah thy God shall choose: one from among thy brethren shalt thou set king over thee; thou mayest not put a foreigner over thee, who is not thy brother." — Deuteronomy 17:15 (ASV)

You shall in any wise set [him] king over you whom the Lord
your God shall choose
The Jews take this to be a command to set a king over them: whereas it is only a permission in case they should desire and determine on having one, as God foresaw they would; and this with a limitation and restriction to appoint none but whom God should choose, and which was their duty and interest to attend to; for none could choose better for them, and was what he has a right to, and it became them to submit to it, since he was their King in a civil and special sense, and another was only his viceregent; accordingly we find, when they expressed their desire to have a king in the time of Samuel, and it was granted, though not without some resentment, the Lord chose their first king for them, Saul, and, after him, David, and even Solomon, David's son.

And though, in later times, they appointed kings without consulting him, it is complained of, (Hosea 8:4) hence this clause is prefaced in the Targum of Jonathan, ``you shall seek instruction from the Lord, and after set him king'' which was to be done by the mouth of a prophet, or by Urim, as Aben Ezra observes.

one from among your brethren shall you set king over you :
that is, one of their own nation, an Israelite, a brother both by nation and religion.

you may not set a stranger over you that is not your brother ;
one of another nation, that is not of the family of Israel, as Aben Ezra notes, even not an Edomite, though called sometimes their brother; and Herod, who was an Idumean, was set up, not by them, but by the Romans; now in this their king was a type of the King Messiah, of whom it is said, "their nobles shall be of themselves", (Jeremiah 30:21) .