What it means
てはいけません expresses that an action is prohibited or not allowed. It is formed by attaching はいけません to the て-form of a verb and is commonly used for rules, instructions, and polite warnings. The familiar counterpart is てはいけない.
Usage note
This form usually states a prohibition rather than personal advice. For softer personal advice, other expressions such as ないほうがいい may be more appropriate.
How to use it
Verb (て-form)はいけません
この部屋で写真を撮ってはいけません。
You must not take pictures in this room.
Verb (て-form)はいけない
この部屋で写真を撮ってはいけない。
You must not take pictures in this room.