Learn Russian: Это значит, что нам снова приходится переносить концерт. - This means that we again have to postpone the concert.

Because приходится is an impersonal verb expressing that circumstances force an action. The person experiencing this forced necessity is always in the Dative case (нам), while the action itself is in the infinitive (переносить).

What is the difference in meaning between нам приходится and мы должны in this context?

While мы должны means we must or we are obliged to (often out of duty), нам приходится means we have to because external circumstances force us to. It emphasizes that postponing the concert is not a choice, but an unavoidable necessity.

Why is the imperfective verb переносить used instead of the perfective перенести?

The word снова (again) indicates that this is a repeated action—they are postponing the concert yet again. In Russian, repeated actions require the imperfective aspect.

Is концерт in the Nominative case here?

It is actually in the Accusative case because it is the direct object of the verb переносить (to postpone the concert). Because концерт is an inanimate masculine noun, its Accusative form is identical to its Nominative form.