Access the Steamworks API from the NW.Js exporter. NOTE: Where possible consider using the Steamworks for WebView2 plugin instead.
Are you sure you're following everything listed here: construct.net/en/make-games/addons/84/greenworks/documentation
I just tested it and it works just fine.
Keep in mind that once you've unlocked an achievement, Steam won't unlock it again. You need to either test a new achievement, or use a 3rd party tool to clear your achievements.