Can you double check please? Im running 5.6.0 on windows and jsonpath count assertion IS available.
However im wondering whether youre trying to use the assertiom on a response with no payload....the assertion wouldnt be available for either xml payload or if there was no payload on your response.
I notice from your screenshot yoy have an enabled xml reaponse tab and there doesnt appear to be any xml content in there (indicating to me your response if it has a payload isnt xml), but if your response had a json type payload and you viewed the xml tab, the text "the content you are trying to view cannot be viewed as xml" would be displayed.
Hence the above deductions make me think youre looking at a response that doesnt include a payload at all!
I hope i was clear!
if this helped answer the post, could you please mark it as 'solved'? Also if you consider whether the title of your post is relevant? Perhaps if the post is solved, it might make sense to update the Subject header field of the post to something more descriptive? This will help people when searching for problems. Ta