Any idea why this is happening?

Tom A. so you have a figma file with the figma kit 3.0 as a library, is that how I'm reading this correctly? My guess is that the plugin is looking for something specific in the actual figma file in which you are trying to import screens into and its not finding it. You'd have to import screens directly into the new figma kit. Once imported, you should be able to move to this other file that you have just fine and since you have the kit as a library, the variables should all be fine too.

Yikes — that’s indeed the answer. Could be worth signposting this better, given it’s a departure from what I’ve been doing previously? Or maybe I’m just being thick, after all 🙃

Tom A. naw you are right - we could have done a much better job at communicating this before the release and I'm all over it for next time. Apologies for the issues here.