Learn Croatian: Ne smijemo ulaziti u crvenu zonu bez pratnje. - We must not enter the red zone without an escort.

This is because of the negative modal ne smijemo (we must not). In Croatian, strict prohibitions almost exclusively pair with imperfective verbs. Even if you are talking about entering just once, the prohibition itself dictates the imperfective ulaziti.

Could we say ne moramo ulaziti instead of ne smijemo ulaziti?

That would completely change the meaning. Ne smijemo means "we must not" or "we are not allowed to" (prohibition). Ne moramo simply means "we don't have to" (lack of obligation). Additionally, if you used ne moramo, you would be free to use the perfective verb ući if you were talking about a single, one-time entry.

Why is the phrase u crvenu zonu in the accusative case here instead of the locative case?

The verb ulaziti (to enter) involves motion toward a destination. When the preposition u indicates direction or moving into something, it always takes the accusative case. If you were already inside the red zone and doing something stationary there, you would use the locative case (u crvenoj zoni).

What case is the word pratnje and what is its base form?