I felt that I would not be able to produce sounds on the flute if I did not enroll myself in a school and take lessons.
こんにちは。
ご質問ありがとうございます。
"I felt that I would not be able to produce sounds on the flute"=「フルートで音を出すことを私には出来ないだろうと感じました」
"if I did not enroll myself in a school and take lessons."=「もしスクールに入ってレッスンを受けなければ。」
ご参考に。