Although
なくてはならない ‘must do’ is introduced to many learners early on,
てはならない by itself carries the meaning of ‘must not do’, or ‘cannot do’. This structure is a combination of the conjunction particle て, は, and the negated form of the う-Verb なる. Despite this ならない by itself is often considered a standalone word which may indicate that something is ‘not possible’, ‘not permitted’, or ‘cannot be helped’, depending on what type of word it is following.
When following verbs and ては, ならない can only mean ‘cannot’, or ‘must not’.
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この池（いけ）では泳（およ）いではならない。
You must not swim in this lake.
お酒（さけ）を飲（の）んで運転（うんてん）してはならない。
You must not drink and drive.
妊娠（にんしん）している間（あいだ）はタバコを吸（す）ってはならない。
You must not smoke while you are pregnant.
Caution - As a reminder, in the order from most casual to most formal, てはだめ,
てはいけない, and
てはならない will often be seen in sentences with no significant difference in meaning.
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こんなことで泣（な）いていてはだめだ！
You shouldn’t cry over something like this.
会議中（かいぎちゅう）に携帯（けいたい）を使（つか）ってはいけない。
You must not use your cellphone during the meeting.
ここから先（さき）は関係者（かんけいしゃ）以外（いがい）、入（はい）ってはならない。
You must not enter from this point forward if you are not an authorized personnel.
Due to the ならない variant being the most formal, it is also the most likely to be seen in writing or polite speech.