Translation:It's fun to sing at Karaoke.
It's rare for kanji to be rejected, and you can usually just submit an error report and it will be added. If it was a "type what you hear" question, though, there's a bug with the duolingo programming that only allows one specific combination of kana and kanji to be accepted as correct, which you can read more about in the comments in this thread on the forum.
The Japanese version does not have the word "song". Maybe the speaker likes to go to karaoke to do some mongolian throat singing.
I put "it is fun singing at karaoke." Duo rejected "singing" in favor of "to sing." But doesn't the "TIP" on this very lesson say that adding "no" has the effect of causing it to "become either “to swim” or “swimming”."???
This one "わたしは本を読むのがきらいです。" looks the same to me, but it rejects "to read" in favor of "reading." Is it arbitrary and Duo is just being mean? Or am I missing something (ALWAYS a possibility)?
It's arbitrary, just need to keep submitting error reports and both the infinitive and the gerund should be accepted. The technical rule is that an adjective is followed by the infinitive (to sing), but as native speakers we often don't follow the rule.
”To sing at a Karaoke is fun"
should this work? And if not, why, what’s wrong with that sentence?
I wouldn't say "a karaoke" in my dialect, but if you say it in yours it's worth an error report.