"C'est un mauvais morceau."
Traduction :It is a bad piece.
I put: It is a wrong piece. Why? That is not good ? I would like to understand.
Some adjectives have positive, comparative and superlative forms.
Bad - worse - worst. (Fr. mauvais - pire - le pire / le plus mauvais).
You use "a" before a positive form, nothing before a comparative form and "the" before a superlative form.
It is a bad piece.
It is worse that the last piece.
It is the worst piece.
Wrong is effectively a superlative - there is nothing "wronger" (not a real word) than wrong, and nothing less wrong than wrong. It's just, well, wrong!
Therefore you must say "... the wrong ....", not "... a wrong ..." (unless wrong is a noun, meaning a tort).
"They played the wrong national anthem at the Olympics." (It is either right or wrong, there is no middle ground.)
"They made a bad decision when selecting the piece." (One of possibly several bad decisions.)
Hopefully, from this explanation, you can understand why your English is wrong and your translation is bad :-)