いけないのよ

mustn't; have to

→ いけない
phrase Rare #36,564
mustn't; have to いけない means “not okay” or “not allowed.” After 〜なきゃ or 〜なくちゃ, the expression means “have to.” のよ adds emphasis or explanation and is conversational, often associated with feminine speech.
JA: ここで写真を撮ったらいけないのよ。
EN: You mustn't take pictures here.

なんでいけないのよ

Why can't I?

— 箪笥<たんす> (2003)

うちらの言うことを聞かなきゃいけないのよ

We have to listen to what our parents say.

勝手にそこへ駐車したらいけないのよ

You mustn't park there without permission.

トム病院に連れて行かなくちゃいけないのよ

I have to take Tom to the hospital.