Lacan's use of the term "instance" goes well beyond Freud's "Instanz".
It represents, one might say, an exploitation of the linguistic possibilities of the French equivalent of Freud's German term.
In the absence of any exact equivalent of Lacan's French term, one is thrown back to the term used by Freud's English translators, "agency".
In Freud, the reference is most often to the three "agencies" of the id, ego and superego.
In Lacan, one must bear in mind the idea of an "acting upon", even "insistence," as in the title of the essay, "L'instance de la letter".