The translation is:
あなたの言[い]ったことがわかりません or 理解[りかい]できません。
*We use (name)さん very often instead of あなた
However, I would say:
よくわからなかったので、もういちどおねがいします。(=Since I didn't/don't understand well, please say it again / once more, please / please repeat it.)
I hope this helps you.