Bennu doesn't have voids as such. But when we look at for example the function free(), we see that you can pass it a void pointer. This means, that you can pass it a pointer of whatever type you want; an int pointer, word pointer or even a pointer pointer. So in this case, void means "undefined".
There is another case in which voids can occur. This is when a function returns nothing, but this never happens in Bennu.