Welcome to the Prismic community, and thanks for reaching out to us.
While looking deeper into your code and JSON configuration, I find that menu_link and menu_label don't come in an array, they are individual objects. You can not apply a loop to get these individual objects.
As I see you want to create a menu with 3rd level items (submenu), I'd suggest you follow this article.
Let me know if you have further questions related to it.