It is briefly mentioned in the notes, but the story goes like this:
- in the more traditional language (as you’ll find in literature), always use càirdean (and a chàirdean when addressing your friends), it is the older traditional plural form of caraid (compare Irish cairde),
- in modern conversational language a new regular plural appeared, and it is common to say caraidean instead of càirdean (like in tha caraidean cudromach friends are important), but still use a chàirdean in vocative to address your friends (eg. an toil leibh e, a chàirdean? do you like it, (my) friends?) – this is what Duolingo teaches,
- you can also encounter a charaidean in vocative but that is rare (because traditional a chàirdean is so commonly used that it sticks in the language) and thus even modern dictionaries don’t acknowledge that usage and thus it’s not accepted on Duolingo.
See eg. that Faclair Dwelly (1920) only mentions càirdean as the plural, but Colin B.D. Mark’s Gaelic-English Dictionary (2003) mentions caraidean as alternative plural often used except for vocative.
So TLDR is: càirdean is older and always used in vocative to address your friends (a chàirdean!) but nowadays often used only in vocative, caraidean is newer and only used outside of vocative.