> 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][1] 

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)

[1]: https://blogs.bl.uk/asian-and-african/2016/03/buddhist-rebirth-in-different-planes-of-existence.html