The Modal Verb Must
We use 'must' to talk about 'rules and obligations' in the present.
The negative form of must is 'must not' or 'mustn’t.' But keep in mind that in modern everyday English, we do not normally use 'mustn't' or 'misu not'.
These sentences are both rather formal and we must use 'shouldn't' or 'should not' instead.
Do not use don’t/doesn’t/didn’t with 'must.'