'Tady nemusíš být' is not being accepted, when I believe it should be.
Seems like "Nemáš tady být" or "Nemáte tady být" should be the correct translation. To me, "Nemusíte tady být" is more like "You must not be here". At the least maybe both are correct?
Using mít plus an infinitive gives you "supposed to." "You are not supposed to be here" is not the same as " You do not have to be here." In the first case, you should not be here. In the second, it is not necessary that you be here.
nemuzes tady byt? is that also ok.
No, that would be you can't.
so nemusis tady byt?
"Nemusíš tady být" is accepted.