Maybe open the Chrome browser in the incognito mode and try to open the Quizlet set, just to be sure that it’s public.
For example, this url works for me if I try to import it with the add-on.
https://quizlet.com/22273412/classroom-japanese-flash-cards/
If you see “‘NoneType’ object has no attribute ‘group’”, try to press “Import Deck” a few more times. I received the error on the first try, but it worked the second time.
