Why is Big O not defined here for a hash table?
The chart is underspecified. I assume they mean by "Access" to "retrieve the $i$-th element¹.

In hashtables, there is no notion of order. While you could pick the $i$-th element in the underlying array, that would have no meaning at all. Hence, this operation does not make sense on hashtables -- n/a.




Which is of course underspecified or unfair, depending on how you look at it: in a BST, "Access" gives you also the $i$-th smallest element, whereas in a plain array you get just some element.