I just took a look at it and from the first grance on the repo, it seems like the plugin was last updated about 1,5 years ago (that was when 1.16 or so was the current release). From that I would guess some changes in the UI department were not incorporated to the plugin and so it does not know where to put the button.
Question to everyone who might take a look at this thread: Is the cockpit plugin officially discontinued (I would not guess so since it is stated nowhere) or just forgotten? I am a little sad about this since we actually thought about implementing cockpit and the plugin just recently.