How to say [I don't Understand what you said] in Japanese?

Question

Answer

言ったことがわかりません, the romanization for this phrase is "itta koto ga wakarimasen" the subject of the phrase is implicit.

Answer

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.