Hello, I just spent like 5 minutes trying to figure this out and figured this might be a common question, so just posting the answer here for posterity. If PieFed hasn’t seen the community before you can add it by navigating to Topics -> All Communities -> Add Remote Community.
It seems like unseen communities don’t automatically get added when you try to migrate your user JSON, so be careful to add any remote communities that are missing.
I got mine to import. I’m not averse to a bit of NSWF content, so have Lemmy set to show NSFW, but most of it did not appeal to me so I just blocked the communities as they came up in my feed. I had 567 blocked communities! Once I edited the JSON to remove those and the import worked.