Actually "この歌だけを歌わなかった。" can be interpreted as "This song was the only one I did not sing", so you can say what you want in the way the guide explains.
And your sentence with しか is correct. Your only mistake is to put だけ after the state of beeing, that's not possible to do.
Actually "この歌だけを歌わなかった。" can
Actually "この歌だけを歌わなかった。" can be interpreted as "This song was the only one I did not sing", so you can say what you want in the way the guide explains.
And your sentence with しか is correct. Your only mistake is to put だけ after the state of beeing, that's not possible to do.