The title of Ubisoft 2016 Watch Dogs 2 is available for free on Epic Games Store, but some players have reported a problem while trying to play the game. After bought the game, we can start downloading it.
However, a problem arises when players try to start the game. They are redirected to Play and when it should automatically activate the game, it requires an activation key instead.
Although the problem is quite boring, the fix is simple.

How

Follow the steps below to solve the activation key problem in Watch Dogs 2.
Close the Play and Epic Games Store client and make sure it doesn’t work in the background.
Go to C: Program Files (X86) Ubisoft Game Launcher Cache, make a backup of the Property folder, then completely delete the cache folder.
Launch Epic Games Store again and launch the game from your library.
You will be asked to link your Play account, follow the steps on the screen to do so.
The game should activate automatically, and the installation should start.
Many players have pointed out that simple restarting their PC also solves the problem.

So, if the above steps do not work for you, you can try to do so too.