"Chan eil mi air bhioran."
Translation:I am not excited.
Had not thought of that, but it would feel exactly the same - not very comfortable. Originally, tenterhooks were for stretching cloth so the phrase meant stressed/tense as much as excited. (Tenter is related to tense and the cloth would be quite literally stressed.) In fact both Mark and AFB give 'on tenterhooks' as one of the meanings. I am not actually familiar with it as meaning 'excited' so it must be a dialect thing.
Generally cha(n) goes with a verb, but there is one exception, and that is is. It only applies to this verb and only in this tense. Here the is is replaced by nothing in the negative and question forms. The historical reason is not fully understood. So
- Tha mi air bhioran → Chan eil mi air bhioran
- Bu toil leam sin → Cha bu toil leam sin
- Is toil leam sin → Cha toil leam sin
I have not seen this discussed here before, but all the evidence from what people report Duolingo counts as an error and what it doesn't points to Duolingo analysing sentences on a word-by-word basis. It splits your sentence at the spaces and then sees if each word is correct. So you should be able to see that even though this is a pretty minor error in term of meaning and pronunciation, it would completely defeat Duolingo. Bad luck.