"I'm bad at the piano."
They went for "bad at piano" instead of "bad at playing the piano" in the Japanese.
You could use the "(verb) no ga" construction in the following way:
(The verb for "play" piano and other string instruments is "pull/pluck": hiku 弾く. Which "play" you use depends on how the instrument is played, so drums are "hit", flutes are "blown", etc. Alternatively, you can use "(musically) perform" for any instrument: 演奏する (えんそうする).)
の would be used to nominalize a verb so in this instance you don't need it. Compare 私はピアノが下手です to 私はピアノをひくのが下手です.
How is ピアノがわたしはへたです。incorrect. Order of proposition doesn't matter in Japanese (but allow to emphasize one part).
My teacher told us that へた was very strong and implies hate so its better to say that you are not skilled じょうずじない. Its a nuance of the language that duolingo misses like when they use あい Instead of 大好き.
Should you add "hiku" so that it's "わたしはピアノひくがへたです。" it would be even more correct with the "のがへた"。
Why is the piano the subject of the sentence here? I'm the one who's bad at piano, not the piano itself. I would have expected watashi ga piano wo heta desu. Or piano wa watashi ga heta desu.
I agree. Been learning Japanese for some time, I've come to the conclusion that this structure is used to describe characteristics or features of people and things. ジョンは目が青いです "John has blue eyes", マリアは背が高いです "Maria is tall", このスマホは画面が大きいです "this smartphone has a large screen", この町は道がせまいです "The streets of this town are narrow", スイスは物価が高いです "The price level of Switzerland is high", 日本は山がきれいです "Mountains in Japan are beautiful."
So "ピアノがへた" = bad at piano but "テニスがじょうず" isn't accepted as "good at tennis". What gives?