What follows at the end of the sentence, after the period ( . ) is usually the .pbo file name that is missing in order to join the mission.

In this case that is exile_client.pbo, indicating a missing/corrupt installation of the Exile modification. The Exile modification is separate from Zombies & Demons.

We are not Exile mod developers, nor are we affiliated with Exile mod developers.

Regardless, I bet you probably just forgot to load up the Exile mod with Arma 3.