I don't know why the other thread with the same topic has been closed when I wrote that "installing T4 Page Builder on a new Joomla instance, it works properly".
That was, instead, another notification of the fact that it does not work (and it still does not know why) on an existing Joomla instace.
Please, re-open the first thread to come to a solution of the problem of using T4 PB on an existing Joomla instance
Thank you
T4 Page Builder does not open
Kindly follow it 
saguaros locked the discussion.