The first level, the Realm of Formlessness (Arupaloka), consists of four planes of brahmas who have no physical body, consisting entirely of mind, but who may create a physical body if they want to be seen. They are not completely free from the fetters of suffering (dukkha), but the dukkha experienced here is much less intense than that suffered in the Rupaloka. These brahmas are unable to hear the teachings of the Buddha (dhamma) and they can never become enlightened.

Buddhist rebirth in different planes of existence

Q1) What is the difference between Arupaloka and Nibbana except Nibbana has no incarnation, no dukkha and has Buddhism teaching ?

Q2) Why brahmas unable to hear the teachings of the Buddha at Arupaloka?

These brahmas are unable to hear the teachings of the Buddha (dhamma)