I know this is an old post but i thought i'd share my enlightening experiences here since there is no solution posted yet (in the end it still wont make any sense...) 😉
When entering "manage API credentials" (2nd last step in the email) you will see all the certificates you ever created in a list. At least in my account i saw more than one certificate there since some old expired certificates were still ghosting around also. Now to mysteriously make the "renew certificate" button appear i removed all the old expired certificates... as i said, this does not make sense but after doing that i could renew my certificate. maybe the same thing can happen to other people too?
...i heard chances are even greater to make the "renew" button appear when shouting the word "paypal" 3 times in a row for every certificate that is beeing removed (works best on full moon nights...).