John Gill Commentary Deuteronomy 17:14

"When thou art come unto the land which Jehovah thy God giveth thee, and shalt possess it, and shalt dwell therein, and shalt say, I will set a king over me, like all the nations that are round about me;" — Deuteronomy 17:14 (ASV)

When you are come unto the land which the Lord your God gives
you
The land of Canaan:

and shall possess it, and shall dwell therein ;
be entirely in the possession of it, and settled in it; it seems to denote some time of continuance in it, as it was, before they thought of setting a king over them, about which are the following instructions:

and shall say, I will set a king over me, like as all the nations
that are round about me ;
which was what would and did lead them to such a thought and resolution; observing that the neighbouring nations had kings over them, they were desirous of being like them as to the form of their civil government, and have a king as they had.