Shouldn't I be able to say एक अच्छा आदमी?
Adding एक is not wrong but it is often dropped.
You can report if the answer was not accepted though.
This sentence is really funny. Sounds like a jury verdict.
Why do you have to have the है here? I thought the नहीं allowed for it to be omitted?
You can omit the है in a negative sentence provided the clause has another verb. In the clause पीटर अच्छा आदमी नहीं है,the only verb is है so it can't be omitted.
I have given the same answer but why its wrong
Why ki and not ka?
This is not the postposition 'की' which is used to indicate possession and has three forms (का,की,के).
This is 'कि' which corresponds to the conjugation 'that' in the English sentence. It does not have any other form.
I wrote ये लोग कह रहे हैं कि पीटर अच्छा नहीं आदमी है but it was marked wrong.
The नहीं has to be placed adjacent to a verb. In this sentence, that would be है.