Yes, this sentence as translated into English makes no sense in English. Is this Gaelic sentence something that does make sense in Gaelic? If so, in English maybe it means "I'm a fun person" or maybe "I'm funny"? To be helpful, these lessons need to give us sentences that make sense in translation and are correct in syntax in both the language being taught and in the native language of the Duolingo learner.
Here the famous Python sketch: https://www.youtube.com/watch?v=oOLS30iByp0 If it makes sense to say "You're not fun", then I'd say it is also OK to reply: (Yes), I am fun. And even the OED has "fun" also as adjective. though this seems to be a more recent development https://www.quickanddirtytips.com/education/grammar/who-says-fun-cant-be-an-adjective
OK, if there's a colloquial dialog going on such as "You're not fun!" "Yes, I am fun"... but that's a rather odd case. And of course "fun" can be an adjective -- I guess I'm just used to it being an adjective applied to activities rather than just being used to say "I am/you are/ s/he is fun."