It has different meaning. ~てはいけません indicates something that shouldn't be done or so. E.g 教室では食べてはいけません - You cannot eat in the classroom ... ない form is just the negative of a verb. Note that ない form conjugates into other forms too such as above. Eg. 食べない- I am not eating 食べなくてはいけません- You/he/I have to eat the food!
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