"Each piece is unique."
Translation:Cada pieza es única.
In the strictest sense, "pedazo" is a correct translation, but really, it doesn't work with this sentence. "Pedazo", as well as "trozo", mean a piece of something larger, like a piece of bread or a piece of a broken plate. "Pieza" means an individual stand alone object, like a piece of art or a specimen. That meaning, i.e. pieza, works much better with this sentence. I think this is what duo is stressing by marking "pedazo" wrong.
For reference: https://www.spanishdict.com/translate/piece
2020-02-21 About a year ago, I was trying to ask a couple of gentlemen whose native language is Spanish about some pieces to a mechanical device. I used las piezas, and they were totally confused. I believe I should have used los pedazos. Fortunately one of them spoke up in English and gave me the information I needed.