beginner's luck
Refers to the phenomenon where a novice unexpectedly succeeds at something, often used in games, sports, or gambling.
初めてのポーカーで勝ったのは、ビギナーズラックだね。
Winning at my first poker game was beginner's luck.
彼のホールインワンはビギナーズラックだった。
His hole-in-one was beginner's luck.