This is an old bug, so do not know if it works, but the last time I even caught, how.
It is a very quick and simple tutorial.
•BuyAssisted in the Premium Shop;
•After that log out from the game;
•Enable it after 24 hours.
So what was done, may have already been fixed or not.
I saw that many people already knew about so I decided to post, even if it does not come complaining, because as I said MAYBE, the fact of having to log in after 24 hours, is that the Assistance is activated automatically after a time, then you want to get the Premium Shop after 24 hours will not help.
In short: Buy, quit the game, come back after 24 hours and ready.