I put, "You don't have to lose her," which was accepted, but it feels like a different shade of meaning, in English at least. "You must not..." sounds like it's going to happen unless you take action to stop it, while "You don't have to..." sounds like it's a choice for the subject. Is må ikke used for both equally?
It can mean both, must not and don't have to. Depends on context, intonation and body language. So because of the ambiguity I'd rather say "Du er ikke nødt til å miste henne" as the don't have to-version. "Å være nødt til" translates rather well into "to have to". Nødt is an adverb in that expression, it doesn't change with gender/number.
But in English you could say still say miss, in the same sense as missing the bus, with people too. For example, if I arrived somewhere and a friend had just left, someone could say to me "Oh you just missed her." So in this sense, "You must not miss her" could still be a valid answer, no? Or do you just never use miste in that way in Norwegian?
When dealing with people you would not use miste unless you are actually holding on to them at the cliff, you are in a relationship and want it to last forever, or if you are in a strange city and you get away from your friends... But the first two means "don't let go of her" and the third "don't get lost from her". None of them fit within "she just left, you missed her".
But you can use miste about appointments and meetings without it sounding weird. When you miss the bus/plane it is because you didn't get there on time, not because you dropped it or can't find it.
More or less copying my own previous answer. The Norwegian statement can mean both, must not and don't have to. Depends on context, intonation and body language. One passive and one active. One emotional and one practical. So we have to allow several English answers to this.