Nation - literally, as in the margin - the word which usually distinguishes the pagan from God’s people. Here it expresses that Israel is cast off by God; and the plural is used to denote that the children of Israel are not even “one nation,” but scattered and disunited.
Translate: “I send thee to the children of Israel, the rebellious nation that have rebelled against Me (they and their fathers have transgressed against Me, even to this very day), and the children impudent and stiff-hearted: I do send thee unto them.”